iris: Add batch-local synchronization book-keeping to iris_bo.

iris: Add batch-local synchronization book-keeping to iris_bo.

The purpose of this is to represent the cache coherency state of a
buffer as a vector of integers (AKA seqnos), one for each incoherent
caching domain of the GPU.  A seqno will identify a single section of
a batch buffer uniquely across the whole pipe_screen (which means that
there will be no ambiguity about what context a given seqno belongs to
even if there are multiple threads accessing the same buffer in
parallel), and is guaranteed to be allocated in monotonically
increasing order within any given context.  The iris_bo_bump_seqno()
helper is provided for marking the last update of a buffer from a
given caching domain in a lockless manner.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3875>