drm/amd/display: Avoid NULL dereference of timing generator

[Why & How]
Check whether assigned timing generator is NULL or not before
accessing its funcs to prevent NULL dereference.
Reviewed-by: Jun Lei <jun.lei@amd.com>
Acked-by: Hersen Wu <hersenxs.wu@amd.com>
Signed-off-by: Wayne Lin <wayne.lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -533,7 +533,7 @@ uint32_t dc_stream_get_vblank_counter(const struct dc_stream_state *stream)
 	for (i = 0; i < MAX_PIPES; i++) {
 		struct timing_generator *tg = res_ctx->pipe_ctx[i].stream_res.tg;
 
-		if (res_ctx->pipe_ctx[i].stream != stream)
+		if (res_ctx->pipe_ctx[i].stream != stream || !tg)
 			continue;
 
 		return tg->funcs->get_frame_count(tg);
@@ -592,7 +592,7 @@ bool dc_stream_get_scanoutpos(const struct dc_stream_state *stream,
 	for (i = 0; i < MAX_PIPES; i++) {
 		struct timing_generator *tg = res_ctx->pipe_ctx[i].stream_res.tg;
 
-		if (res_ctx->pipe_ctx[i].stream != stream)
+		if (res_ctx->pipe_ctx[i].stream != stream || !tg)
 			continue;
 
 		tg->funcs->get_scanoutpos(tg,