drm/radeon/kms: add wait idle ioctl for eg->cayman

None of the latest GPUs had this hooked up, this is necessary for
correct operation in a lot of cases, however we should test this on a few
GPUs in these families as we've had problems in this area before.

Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>