asahi: Do not use memctx for pools / meta cache

asahi: Do not use memctx for pools / meta cache

ralloc is not thread-safe, so we can't use dev->memctx for allocating
context-specific things without locking. On top of that, we always
need to explicitly clean up pools anyway since we need to unref the BOs,
so there is no point to using a memctx.

And since pools need to be explicitly cleaned up, the meta cache code
needs explicit cleanup, so add that and drop memctx from there too.

Signed-off-by: Asahi Lina <lina@asahilina.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21348>