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 <[email protected]>
Acked-by: Hersen Wu <[email protected]>
Signed-off-by: Wayne Lin <[email protected]>
Tested-by: Daniel Wheeler <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>

Author: Wayne Lin

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,