drm: Release user fbs in drm_release

Avoids leaking fbs and associated buffers on release.

Signed-off-by: Kristian Høgsberg <krh@redhat.com>
Tested-by: Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@linux.ie>