drm/i915/icl: split combo and mg pll enable

Let's start using the vfuncs to differentiate MG and Combo PLLs. The end
goal is to decouple the type of the PLL from the IDs since the latter
are likely to change from one platform to another. This also makes the
code easier to read by not having lots of if/else chains on leaf
functions.

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/20190309035727.25389-2-lucas.demarchi@intel.com