drm/i915/icl: move MG pll hw_state readout

Let the MG plls have their own hooks since it shares very little with
other PLL types. It's also better so the platform info contains the info
if the PLL is for MG PHY rather than relying on the PLL ids.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190222232324.16405-1-lucas.demarchi@intel.com