panfrost: Allocate dedicated slab for linear BOs

panfrost: Allocate dedicated slab for linear BOs

Previously, linear BOs shared memory with each other to minimize kernel
round-trips / latency, as well as to work around a bug in the free_slab
function. These concerns are invalid now, but continuing to use the slab
allocator for BOs resulted in memory allocation errors. This issue was
aggravated, though not introduced (so not a real regression) in the
previous commit.

v2 (unreviewed): Fix bug in v1 preventing munmaps from working

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>