freedreno: add bo cache

Workloads which create many transient buffers cause significant CPU
overhead in buffer allocation, zeroing, cache maint, and mmap setup.
By caching and re-using existing buffers, the CPU overhead drops
significantly.  See:

http://bloggingthemonkey.blogspot.com/2013/09/freedreno-update-moar-fps.html

A simple time based policy is used for purging the cache.  Once the
kernel supports it, we could use madvise style API to handle memory
pressure scenarios a bit better.

Signed-off-by: Rob Clark <robclark@freedesktop.org>