drm/radeon: fix regression in dynpm due to multi-ring rework

Not all asics have all rings, so make sure the ring is ready
before attempting to check it in the dynpm work handler.

Fixes:
https://bugzilla.kernel.org/show_bug.cgi?id=43367

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>