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

Like was done in the enable case, split the implementation of the
disable for MG and Combo PLLs.

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/20190309035727.25389-4-lucas.demarchi@intel.com