[magma] Revert to 32 bit gem handles.

In mesa 20.0 anvil is doubling down on the assumption of 32 bit
gem handles.  Maintaining 64 bit handles (pointers) on Fuchsia
causes too much merge headache, so before merging 20.0 start vending
32 bit handles and map them to magma buffers using mesa's upcoming
threadsafe sparse array.

Change-Id: I8fad50456bfa47274669d55c2cd857cedbf719d0
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/399928
Reviewed-by: John Bauman <jbauman@google.com>
7 files changed