drm/amd/display: Fix null pointer

Add guard for NULL pointer access
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Acked-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Johnson Chen <johnson.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dml2/dml2_utils.c

@@ -158,6 +158,8 @@ bool is_dp2p0_output_encoder(const struct pipe_ctx *pipe_ctx)
 	/* If this assert is hit then we have a link encoder dynamic management issue */
 	ASSERT(pipe_ctx->stream_res.hpo_dp_stream_enc ? pipe_ctx->link_res.hpo_dp_link_enc != NULL : true);
 
+	if (pipe_ctx->stream == NULL)
+		return false;
 	/* Count MST hubs once by treating only 1st remote sink in topology as an encoder */
 	if (pipe_ctx->stream->link && pipe_ctx->stream->link->remote_sinks[0]) {
 			return (pipe_ctx->stream_res.hpo_dp_stream_enc &&