drm/i915: Program pch transcoder m2/n2

Program the PCH transcoder M2/N2 values appropriately. We're
still missing a few things for PCH port DRRS but at least this
means we can do readout/state check for dp_m2_n2 unconditionally.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220128103757.22461-15-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>