drm/i915/mst: Disable transcoder before deleting the payload

Bspec tells us that we should disable the transcoder before
deleting the payload. Looks like this has been reversed since
MST support was added.

I suppose this shouldn't matter in practice since the downstream
device shouldn't really do anything with the new payload until
we send the ACT. But I see no compelling reason to deviate from
the bspec sequence regardless.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231018154123.5479-3-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>