drm/ttm: add a wrapper for checking if manager is in use

This converts vmwgfx over to using an interface to set the
in use and check the in use flag.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200804025632.3868079-57-airlied@gmail.com