drm/amd/display: Check for NULL pointer

[why & how]
Need to make sure plane_state is initialized
before accessing its members.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Reviewed-by: Xi (Alex) Liu <xi.liu@amd.com>
Signed-off-by: Sung Joon Kim <sungjoon.kim@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/core/dc_surface.c

@@ -143,6 +143,7 @@ const struct dc_plane_status *dc_plane_get_status(
 		if (pipe_ctx->plane_state != plane_state)
 			continue;
 
+		if (pipe_ctx->plane_state)
 			pipe_ctx->plane_state->status.is_flip_pending = false;
 
 		break;