drm/bufs: remove handling of _DRM_GEM mappings

Gone with the new gem vma offset manager from David.

We can also ditch the uapi header definition from the enum since
userspace never used this. It ended up in there purely for historical
reasons (for reusing the old drm mmap code essentially), not because
userspace ever needed it.

Cc: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>