drm/i915/mst: use intel_de_rmw() to simplify VC payload alloc set/clear

Less is more, fewer lines to wonder about.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210813115610.20010-1-jani.nikula@intel.com