drm/dp_mst: Fix modeset tracking in drm_dp_atomic_release_vcpi_slots()

Currently with the MST helpers we avoid releasing payloads _and_ avoid
pulling in the MST state if there aren't any actual payload changes. While
we want to keep the first step, we need to now make sure that we're always
pulling in the MST state on all modesets that can modify payloads - even if
the resulting payloads in the atomic state are identical to the previous
ones.

This is mainly to make it so that if a CRTC is still assigned to a
connector but is set to DPMS off, the CRTC still holds it's payload
allocation in the atomic state and still appropriately pulls in the MST
state for commit tracking. Doing this allows for us to still track
bandwidth limitations in a state correctly even between DPMS changes, so
that there's no chance of a simple ->active change being rejected by the
atomic check.

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