loader/dri3: Use dri3_wait_for_event_locked in loader_dri3_wait_for_msc

loader/dri3: Use dri3_wait_for_event_locked in loader_dri3_wait_for_msc

Before, if one thread ended up waiting in dri3_wait_for_event_locked
and another one in loader_dri3_wait_for_msc at the same time, one thread
could end up processing an event the other thread was waiting for, which
could result in the latter thread waiting longer than necessary
(possibly indefinitely).

Noticed by inspection.

v2:
* Drop xcb_flush call from loader_dri3_wait_for_msc in favour of the one
  in dri3_wait_for_event_locked (Kenneth Graunke)

Fixes: 7b0e8264dd21 "loader/dri3: Try to make sure we only process our
                     own NotifyMSC events"

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5368>