drm/amdgpu/dm/mst: Stop grabbing mst_mgr->lock in compute_mst_dsc_configs_for_state()

Noticed this while trying to update amdgpu for the non-atomic MST removal
changes - for some reason we appear to grab mst_mgr->lock before computing
mst DSC configs. This is wrong though - mst_mgr->lock is only needed while
traversing the actual MST topology state - which is not typically something
that DRM drivers should be doing themselves anyway.

Signed-off-by: Lyude Paul <lyude@redhat.com>