drm/i915: Split ICL MG PHY buf trans per output type

Make the mess inside the buf trans funcs a bit more manageable by
splitting along the lines of output type.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200929233449.32323-7-ville.syrjala@linux.intel.com


Reviewed-by: Imre Deak <imre.deak@intel.com>