gallium/threaded_context: add another rule for buffer mapping

gallium/threaded_context: add another rule for buffer mapping

a synchronous driver can use PIPE_MAP_ONCE to infer that a buffer is
guaranteed to not be mapped multiple times, as this is only used when
doing map -> memcpy -> unmap directly

a threaded driver performs maps/unmaps asynchronously, so this flag
can only be used by the driver to confirm that the mapped region is accessed
exactly once, not that it will not need to remain mapped for other transfer_map
uses after it is unmapped

in short, consider this scenario:
  transfer_map(A) -> memcpy(map, data) -> transfer_unmap(map_A) ->
  transfer_map(A) -> memcpy(map, data) -> transfer_unmap(map_A)

when a synchronous driver executes this, the call chain is unmodified

when a tc driver executes this, the call chain may become:
  transfer_map(A) -> memcpy(map, data) ->
  transfer_map(A) -> memcpy(map, data) ->
  transfer_unmap(map_A) -> transfer_unmap(map_A)

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10113>