Allocating BOs is expensive, so we should avoid doing that by caching
freed BOs.

BO cache is modelled after one in v3d driver and works as follows:

- in lima_bo_create() check if we have matching BO in cache and return
  it if there's one, allocate new BO otherwise.
- in lima_bo_unreference() (renamed from lima_bo_free()): put BO in
  cache instead of freeing it and remove all stale BOs from cache

Reviewed-by: Qiang Yu <yuq825@gmail.com>
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>