drm/radeon: set the full cache bit for fences on r7xx+

commit d45b964a22cad962d3ede1eba8d24f5cee7b2a92 upstream.

Needed to properly flush the read caches for fences.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>