drm/i915/mst: Clear ACT just before triggering payload allocation

Follow the bspec sequence more closely and clear ACT sent just
before triggering the allocation. Can't see why we'd want to
deviate from the spec sequence here.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231018154123.5479-4-ville.syrjala@linux.intel.com


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