gallium/pb_cache: add a copy of cache bufmgr independent of pb_manager

gallium/pb_cache: add a copy of cache bufmgr independent of pb_manager

This simplified (basically duplicated) version of pb_cache_manager will
allow removing some ugly hacks from radeon and amdgpu winsyses and
flatten simplify their design.

The difference is that winsyses must manually add buffers to the cache
in "destroy" functions and the cache doesn't know about the buffers before
that. The integration is therefore trivial and the impact on the winsys
design is negligible.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Acked-by: Michel Dänzer <michel.daenzer@amd.com>