drm/radeon: Drop legacy MST support

drm/radeon: Drop legacy MST support

Right now, radeon is technically the only non-atomic driver still making
use of the MST helpers - and thus the final user of all of the legacy MST
helpers. Originally I was going to look into seeing if we could move legacy
MST into the radeon driver itself, however:

* SI and CIK both can use amdgpu, which still supports MST
* It currently doesn't work according to my own testing. I'm sure with some
  troubleshooting we could likely fix it, but that brings me to point #2:
* It was never actually enabled by default, and is still marked as
  experimental in the module parameter description
* If people were using it, someone probably would have probably seen a bug
  report about how it is currently not functional by now. That certainly
  doesn't appear to be the case, since before getting access to my own
  hardware I had to go out of my way to try finding someone to help test
  whether this legacy MST code even works - even amongst AMD employees.
* Getting rid of this code and only having atomic versions of the MST
  helpers to maintain is likely going to be a lot easier in the long run,
  and will make it a lot easier for others contributing to this code to
  follow along with what's happening.

FWIW - if anyone still wants this code to be in the tree and has a good
idea of how to support this without needing to maintain the legacy MST
helpers (trying to move them would probably be acceptable), I'm happy to
suggestions. But my hope is that we can just drop this code and forget
about it. I've already run this idea by Harry Wentland and Alex Deucher a
few times as well.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Wayne Lin <Wayne.Lin@amd.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Fangzhi Zuo <Jerry.Zuo@amd.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sean Paul <sean@poorly.run>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220817193847.557945-17-lyude@redhat.com