drm/i915/dsc/mtl: Enable alternate ICH method

DSC 1.2 is supported from MTL, hence program ICH accordingly.

Cc: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220817124516.284456-2-jani.nikula@intel.com