drm/radeon: Deprecate UMS support v2

KMS support is out and stable for a couple of years now and
the userspace code has deprecated or abandoned the old UMS interface.

So make the KMS interface the default and deprecate the UMS interface
in the kernel as well.

v2: rebased on alex/drm-next-3.9-wip

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>