pan/decode: Use a page table for tracking mmaps

pan/decode: Use a page table for tracking mmaps

We create a hash table mapping GPU va's to mmap structures, such that
searching for a mapped address is effectively O(1) rather than O(N) to
the number of mapped entries as with the previous linked list approach.
This is a memory-time tradeoff, but the speed-up is tracing is notable.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5099>