clover: Fix a bug with multi-threaded events v2

clover: Fix a bug with multi-threaded events v2

It was possible for some events never to get triggered if one thread
was creating events and another threads was waiting for them.

This patch consolidates soft_event::wait() and hard_event::wait()
into event::wait() so that hard_event objects will now wait for
all their dependencies to be submitted before flushing the command
queue.

v2:
  - Rename variables
  - Use mutable varibales so we can keep event::wait() const
  - Open code signalled() call so mutex can be atted to signalled
    without deadlocking.

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