drm/i915/icl: split combo and tbt pll funcs

Like was done for MG and combo, now finish the per-type split of the
vfunc by moving TBT out of the combo functions. Now we can completely
remove icl_pll_id_to_enable_reg() since each PLL type passes all the
information via arguments.

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/20190309035727.25389-5-lucas.demarchi@intel.com