drm/amdgpu/dm/mst: Use the correct topology mgr pointer in amdgpu_dm_connector

This bug hurt me. Basically, it appears that we've been grabbing the
entirely wrong mutex in the MST DSC computation code for amdgpu! While
we've been grabbing:

  amdgpu_dm_connector->mst_mgr

That's zero-initialized memory, because the only connectors we'll ever
actually be doing DSC computations for are MST ports. Which have mst_mgr
zero-initialized, and instead have the correct topology mgr pointer located
at:

  amdgpu_dm_connector->mst_port->mgr;

I'm a bit impressed that until now, this code has managed not to crash
anyone's systems! It does seem to cause a warning in LOCKDEP though:

  [   66.637670] DEBUG_LOCKS_WARN_ON(lock->magic != lock)

This was causing the problems that appeared to have been introduced by:

  commit 4d07b0bc ("drm/display/dp_mst: Move all payload info into the atomic state")

This wasn't actually where they came from though. Presumably, before the
only thing we were doing with the topology mgr pointer was attempting to
grab mst_mgr->lock. Since the above commit however, we grab much more
information from mst_mgr including the atomic MST state and respective
modesetting locks.

This patch also implies that up until now, it's quite likely we could be
susceptible to race conditions when going through the MST topology state
for DSC computations since we technically will not have grabbed any lock
when going through it.

So, let's fix this by adjusting all the respective code paths to look at
the right pointer and skip things that aren't actual MST connectors from a
topology.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Gitlab issue: drm/amd#2171
Fixes: 8c20a1ed ("drm/amd/display: MST DSC compute fair share")
Cc: <stable@vger.kernel.org> # v5.6+