drm/amd/display: Check dce_hwseq before dereferencing it

[WHAT]

hws was checked for null earlier in dce110_blank_stream, indicating hws
can be null, and should be checked whenever it is used.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 79db436)
Cc: stable@vger.kernel.org

Author: Alex Hung

drivers/gpu/drm/amd/display/dc/hwss/dce110/dce110_hwseq.c

@@ -1225,7 +1225,7 @@ void dce110_blank_stream(struct pipe_ctx *pipe_ctx)
 		return;
 
 	if (link->local_sink && link->local_sink->sink_signal == SIGNAL_TYPE_EDP) {
-		if (!link->skip_implict_edp_power_control)
+		if (!link->skip_implict_edp_power_control && hws)
 			hws->funcs.edp_backlight_control(link, false);
 		link->dc->hwss.set_abm_immediate_disable(pipe_ctx);
 	}