Previously, we allocated a single 256MB slab of texture memory on init,
never freed anything, and hoped it was enough. This is bad on so many
levels.

This patch uses the pb_slab interface courtesy of AMD to manage texture
memory. With the new API, we don't allocate any slabs on startup; we
instead provide a callback for allocating a new slab of memory, which
we implement with the existing kbase API. Then, we use its methods to
allocate and free texture memory.

For real-world motivation, without this patch, running
sway+es2gears (via Panfrost/llvmpipe respectively) would allocate 50% of
my memory, and then crash after a short amount of time due to OOM
conditions. With this patch, the same combination allocates about 30% of
memory and can run essentially indefinitely, since we're freeing
resources appropriately.

Future patches will extend this interface for other parts of the
cmdstream to further reduce our memory footprint.