drm/etnaviv: add lockdep annotation for userptr object population

The current userptr page population will defer work to a work item if
needed to avoid ever taking the mmap_sem in the direct call path. With
the more fine-grained locking in etnaviv this isn't needed anymore, so
a future commit will simplify this code.

Add a lockdep annotation to validate the assumption that the mmap_sem
can be taken in the direct call path.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>