We were already relying on pScrn->pScreen being non-NULL in some cases,
which is supposedly always true ever since this function is no longer
getting called from ScreenInit.
(Ported from radeon commit eb611a2e4ecce7a1ab85fd72b9b78e3269311dd5)
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>