drm/dp_mst: Don't open code modeset checks for releasing VC slots

I'm not sure why, but at the time I originally wrote the find/release VC
slot helpers I thought we should avoid keeping modeset tracking out of the
MST helpers. In retrospect though there's no actual good reason to do
this, and the logic has ended up being identical across all the drivers
using the helpers. Also, it needs to be fixed anyway so we don't break
things when going atomic-only with MST.

So, let's just move this code into drm_dp_atomic_release_vcpi_slots() and
stop open coding it.

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