drm/amd/display: added NULL check at start of dc_validate_stream

[Why]
prevent invalid memory access

[How]
check if dc and stream are NULL
Co-authored-by: winstang <winstang@amd.com>
Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Acked-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Signed-off-by: winstang <winstang@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_resource.c

@@ -4754,6 +4754,9 @@ void resource_build_bit_depth_reduction_params(struct dc_stream_state *stream,
 
 enum dc_status dc_validate_stream(struct dc *dc, struct dc_stream_state *stream)
 {
+	if (dc == NULL || stream == NULL)
+		return DC_ERROR_UNEXPECTED;
+
 	struct dc_link *link = stream->link;
 	struct timing_generator *tg = dc->res_pool->timing_generators[0];
 	enum dc_status res = DC_OK;