drm/amd/display: Fix dereference-before-check for dc_link

The function dereferences amdgpu_dm_connector->dc_link early to
initialize verified_link_cap and dc, but later still checks
amdgpu_dm_connector->dc_link for NULL in the analog path.

This late NULL check is redundant, introduce a local dc_link pointer,
use it consistently, and drop the superfluous NULL check while using
dc_link->link_id.id instead.

The function uses dc_link at the very beginning without checking if it
is NULL.  But later in the code, it suddenly checks if dc_link is NULL.

This check is too late to be useful, because the code has already used
dc_link earlier.  So this NULL check does nothing.

We simplify the code by storing amdgpu_dm_connector->dc_link in a local
dc_link variable and using it throughout the function.  Since dc_link is
already dereferenced early, the later NULL check is unnecessary and is
removed.

Fixes the below:
  amdgpu_dm_connector_get_modes():
  variable dereferenced before check 'amdgpu_dm_connector->dc_link'

drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c
  8845				&amdgpu_dm_connector->dc_link->verified_link_cap;
  8846		const struct dc *dc = amdgpu_dm_connector->dc_link->dc;
                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                                      Dereference
  ...

  8856
  8857			if (amdgpu_dm_connector->dc_sink &&
  8858			    amdgpu_dm_connector->dc_link &&
                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                            Checked too late.
                            Presumably this NULL check could be removed?
  ...

Fixes: d46e422 ("drm/amd/display: Cleanup uses of the analog flag")
Reported by: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Timur Kristóf <timur.kristof@gmail.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Roman Li <roman.li@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Tom Chung <chiahsuan.chung@amd.com>
Cc: Alex Hung <alex.hung@amd.com>
Cc: Ivan Lipski <ivan.lipski@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

Author: srishanm

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -8854,11 +8854,11 @@ static int amdgpu_dm_connector_get_modes(struct drm_connector *connector)
 {
 	struct amdgpu_dm_connector *amdgpu_dm_connector =
 			to_amdgpu_dm_connector(connector);
+	struct dc_link *dc_link = amdgpu_dm_connector->dc_link;
 	struct drm_encoder *encoder;
 	const struct drm_edid *drm_edid = amdgpu_dm_connector->drm_edid;
-	struct dc_link_settings *verified_link_cap =
-			&amdgpu_dm_connector->dc_link->verified_link_cap;
-	const struct dc *dc = amdgpu_dm_connector->dc_link->dc;
+	struct dc_link_settings *verified_link_cap = &dc_link->verified_link_cap;
+	const struct dc *dc = dc_link->dc;
 
 	encoder = amdgpu_dm_connector_to_encoder(connector);
 
@@ -8870,9 +8870,8 @@ static int amdgpu_dm_connector_get_modes(struct drm_connector *connector)
 				drm_add_modes_noedid(connector, 1920, 1080);
 
 		if (amdgpu_dm_connector->dc_sink &&
-		    amdgpu_dm_connector->dc_link &&
 		    amdgpu_dm_connector->dc_sink->edid_caps.analog &&
-		    dc_connector_supports_analog(amdgpu_dm_connector->dc_link->link_id.id)) {
+		    dc_connector_supports_analog(dc_link->link_id.id)) {
 			/* Analog monitor connected by DAC load detection.
 			 * Add common modes. It will be up to the user to select one that works.
 			 */