commit d45b964a upstream.

Needed to properly flush the read caches for fences.
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
[bwh: Backported to 3.2:
 - Adjust context
 - s/\bring\b/rdev/]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>