drm/amdgpu/dm/dp_mst: Don't grab mst_mgr->lock when computing DSC state

Now that we've fixed the issue with using the incorrect topology manager,
we're actually grabbing the topology manager's lock - and consequently
deadlocking. Luckily for us though, there's actually nothing in AMD's DSC
state computation code that really should need this lock. The one exception
is the mutex_lock() in dm_dp_mst_is_port_support_mode(), however we grab no
locks beneath &mgr->lock there so that should be fine to leave be.

Gitlab issue: https://gitlab.freedesktop.org/drm/amd/-/issues/2171



Signed-off-by: Lyude Paul <lyude@redhat.com>
Fixes: 8c20a1ed ("drm/amd/display: MST DSC compute fair share")
Cc: <stable@vger.kernel.org> # v5.6+
Reviewed-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>