drm/i915: Always check dp_m2_n2 on pre-bdw

No point in special casing the check of dp_m2_n2 on pre-bdw platforms.
Either the transcoder has M2/N2 in which case the values should be
set to something sensible, or it doesn't in which case dp_m2_n2 is
always zeroed.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220128103757.22461-17-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>