u_transfer_helper: flush temporary transfer first for MSAA

u_transfer_helper: flush temporary transfer first for MSAA

vtbl->transfer_map may return staging buffer and not real one and it
exposes a problem in MSAA resolve path, since u_transfer_helper does
blit from a resource that is still mapped and it's not flushed yet.

Add explicit flush_region() for a temporary transfer before doing flush
for MSAA resolve.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16923>