clover: Add a mutex to guard queue::queued_events

clover: Add a mutex to guard queue::queued_events

This fixes a potential crash where on a sequence like this:

Thread 0: Check if queue is not empty.
Thread 1: Remove item from queue, making it empty.
Thread 0: Do something assuming queue is not empty.

CC: 10.5 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>