drm/amd/display: Fix null check for pipe_ctx->plane_state in hwss_setup_dpp

This commit addresses a null pointer dereference issue in
hwss_setup_dpp(). The issue could occur when pipe_ctx->plane_state is
null. The fix adds a check to ensure `pipe_ctx->plane_state` is not null
before accessing. This prevents a null pointer dereference.

Fixes: 0baae62 ("drm/amd/display: Refactor fast update to use new HWSS build sequence")
Reviewed-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Zicheng Qu <quzicheng@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

Author: Zicheng Qu

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

@@ -898,6 +898,9 @@ void hwss_setup_dpp(union block_sequence_params *params)
 	struct dpp *dpp = pipe_ctx->plane_res.dpp;
 	struct dc_plane_state *plane_state = pipe_ctx->plane_state;
 
+	if (!plane_state)
+		return;
+
 	if (dpp && dpp->funcs->dpp_setup) {
 		// program the input csc
 		dpp->funcs->dpp_setup(dpp,