drm/amd/display: check dc_link before dereferencing

drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:6683 amdgpu_dm_connector_funcs_force()
warn: variable dereferenced before check 'dc_link' (see line 6663)

Fixes: 9671761 ("drm/amd/display: fix null-pointer dereference on edid reading")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

Author: melissawen

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

@@ -6539,7 +6539,7 @@ static void amdgpu_dm_connector_funcs_force(struct drm_connector *connector)
 	struct edid *edid;
 	struct i2c_adapter *ddc;
 
-	if (dc_link->aux_mode)
+	if (dc_link && dc_link->aux_mode)
 		ddc = &aconnector->dm_dp_aux.aux.ddc;
 	else
 		ddc = &aconnector->i2c->base;