Revert "radeonsi: avoid syncing the driver thread in si_fence_finish"

This reverts commit bc65dcab.

This was manually reverted. Reverting stops the menu hanging in
some id tech games such as RAGE and Wolfenstein The New Order.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107891
(cherry picked from commit 7a992fcf)