drm/amd/display: Check for NULL pointer

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

Cc: Mario Limonciello <[e-mail]>
Cc: Alex Deucher <[e-mail]>
Cc: [e-mail]
Reviewed-by: Xi (Alex) Liu <[e-mail]>
Signed-off-by: Sung Joon Kim <[e-mail]>
Signed-off-by: Aurabindo Pillai <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

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

···
 		if (pipe_ctx->plane_state != plane_state)
 			continue;
 
-		pipe_ctx->plane_state->status.is_flip_pending = false;
+		if (pipe_ctx->plane_state)
+			pipe_ctx->plane_state->status.is_flip_pending = false;
 
 		break;
 	}