drm: Kill DRM_SUSER

Checking directly for the right capability is simpler. Also this rids
us of a few places that use DRM_CURRENTPID.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>

diff --cc drivers/gpu/drm/ttm/ttm_bo_util.c
Simple merge

diff --cc drivers/gpu/drm/via/via_dma.c
Simple merge

diff --cc include/drm/drm_os_linux.h
Simple merge