Revert "drm/vgem: drop DRM_AUTH usage from the driver"

This reverts commit e4eee93d.

Mandatory review was missing from this patch.

Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Acked-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190807142101.251400-4-sean@poorly.run