drm/etnaviv: split obj locks in different classes depending on the obj type

Userptr, prime and shmem buffer objects have different lock ordering
requirements. This is mostly due to the fact that we don't allow to mmap
userptr buffers, so we won't ever end up in our fault handler for those,
so some of the code paths are never called with the mmap_sem held.

To avoid lockdep false positives, split them up into different lock classes.

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