drm/i915: Extract intel_{get,set}_m_n()

Make the M/N setup/readout a bit less repitive by extracting
a few small helpers.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220127093303.17309-2-ville.syrjala@linux.intel.com


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