drm/i915/dp_mst: Enable MST DSC decompression for all streams

Enable DSC decompression for all streams. In particular atm if a sink is
connected to a last branch device that is downstream of the first branch
device connected to the source, decompression is not enabled for it.
Similarly it's not enabled if the sink supports this with the last
branch device passing through the compressed stream to it.

Enable DSC in the above cases as well. Since last branch devices may
handle the decompression for multiple ports, toggling DSC needs to be
refcounted, add this using the DSC AUX device as a reference.

v2:
- Fix refcounting, setting/clearing
  connector->dp.dsc_decompression_enabled always as needed. (Stan)
- Make the refcounting more uniform for the SST vs. MST case.
- Add state checks for connector->dp.dsc_decompression_enabled and
  connector crtc.
- Sanitize connector DSC decompression state during HW setup.
- s/use_count/ref_count/
v3:
- Remove stale TODO: comment to set the actual decompression_aux.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231107001505.3370108-6-imre.deak@intel.com