drm/amd/display: Check phantom_stream before it is used

dcn32_enable_phantom_stream can return null, so returned value
must be checked before used.

This fixes 1 NULL_RETURNS issue reported by Coverity.
Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Jerry Zuo <jerry.zuo@amd.com>
Signed-off-by: Alex Hung <alex.hung@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/resource/dcn32/dcn32_resource.c

@@ -1717,6 +1717,9 @@ void dcn32_add_phantom_pipes(struct dc *dc, struct dc_state *context,
 	// be a valid candidate for SubVP (i.e. has a plane, stream, doesn't
 	// already have phantom pipe assigned, etc.) by previous checks.
 	phantom_stream = dcn32_enable_phantom_stream(dc, context, pipes, pipe_cnt, index);
+	if (!phantom_stream)
+		return;
+
 	dcn32_enable_phantom_plane(dc, context, phantom_stream, index);
 
 	for (i = 0; i < dc->res_pool->pipe_count; i++) {