drm/dp_mst: Rename drm_dp_mst_vcpi_allocation

In retrospect, the name I chose for this originally is confusing, as
there's a lot more info in here then just the VCPI. This really should be
called a payload. Let's make it more obvious that this is meant to be
related to the atomic state and is about payloads by renaming it to
drm_dp_mst_atomic_payload. Also, rename various variables throughout the
code that use atomic payloads.

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