drm/radeon/cik: Add support for compute queues (v4)

On CIK, the compute rings work slightly differently than
on previous asics, however the basic concepts are the same.

The main differences:
- New MEC engines for compute queues
- Multiple queues per MEC:
  - CI/KB: 1 MEC, 4 pipes per MEC, 8 queues per pipe = 32 queues
  -    KV: 2 MEC, 4 pipes per MEC, 8 queues per pipe = 64 queues
- Queues can be allocated and scheduled by another queue
- New doorbell aperture allows you to assign space in the aperture
  for the wptr which allows for userspace access to queues

v2: add wptr shadow, fix eop setup
v3: fix comment
v4: switch to new callback method

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>