drm/radeon: add WAIT_UNTIL to evergreen VM safe reg list

commit 668bbc81baf0f34df832d8aca5c7d5e19a493c68 upstream.

It's used in a recent mesa commit:
http://cgit.freedesktop.org/mesa/mesa/commit/?id=24b1206ab2dcd506aaac3ef656aebc8bc20cd27a
and there may be some other cases in the future where it's required.

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