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

src/gallium/drivers/swr/swr_fence.cpp

@@ -50,7 +50,9 @@ swr_fence_cb(uint64_t userData, uint64_t userData2, uint64_t userData3)
    swr_fence_do_work(fence);
 
    /* Correct value is in SwrSync data, and not the fence write field. */
-   fence->read = userData2;
+   /* Contexts may not finish in order, but fence value always increases */
+   if (fence->read < userData2)
+      fence->read = userData2;
 }
 
 /*