xa: don't leak fences

xa: don't leak fences

XA was never unref'ing last_fence in the various call paths to
pipe->flush().  Add this to xa_context_flush() and update the other
open-coded calls to pipe->flush() to use xa_context_flush() instead.

This fixes a memory leak reported with xf86-video-freedreno.

Reported-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>
Cc: "10.5 10.6" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Rob Clark <robclark@freedesktop.org>