The dma ring can't write to register thus have to write to memory
its fence value. This ensure that it doesn't try to use scratch
register for dma ring fence driver.

Should fix:
https://bugs.freedesktop.org/show_bug.cgi?id=58166Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>