drm/radeon: disable vce init on cayman (v2)

Cayman does not have vce.  There were a few places in the
shared cayman/TV code where we were trying to do vce stuff.

v2: remove -ENOENT check

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