drm/i915: Nuke intel_dp_get_m_n()

As with intel_dp_set_m_n() let's get rid of the wrapper and just
call the relevant PCH vs. CPU transcoder functions directly.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220128103757.22461-3-ville.syrjala@linux.intel.com


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