panfrost: Get rid of the now unused SLAB allocator

The last users have been converted to use plain BOs. Let's get rid of
this abstraction. We can always consider adding it back if we need it
at some point.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>