gallium/swr: Fix multi-context sync fence deadlock.

Various recreation scenarios lead to API thread getting stuck in
swr_fence_finish().  This is a multi-context issue, whereby one context
overwrites the fence read-value with a previous sync's lesser value.
The fence sync value is supposed to be always increasing.

In swr_fence_cb(), only update the "read" value if the new value is
greater.

(This may seem like we're not waiting on the other context to finish, but
had we needed for it to finish there would have been a wait prior to
submitting a new sync.)

cc: mesa-stable@lists.freedesktop.org
(cherry picked from commit ed7673af)