[Intel-gfx] [RFC] Try harder to allocate an mmap offset

[Chris Wilson]

An issue that remains unresolved is the exhaustion of the mmap address
space through fragmentation and the persistence of lots of bo (usually
found in the libdrm cache of the xf86-video-intel). This series tries to
improve matters by evicting purgeable objects to free up mmap space when
we fail to allocate a new offset.
-Chris