drm/i915/icl: reduce pll_id scope and use enum type

Now that pll_id is not used anymore for combophy, reduce its scope.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190322223751.22089-6-lucas.demarchi@intel.com


Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>