drm/i915: Split intel_cpu_transcoder_get_m_n() into M1/N1 vs. M2/N2 variants

As with intel_cpu_transcoder_set_m_n() let's split the readout
counterpart into explicit M1/N1 vs. M2/N2 variants as well.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220128103757.22461-6-ville.syrjala@linux.intel.com


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