drm/i915/icl: remove intel_dpll_is_combophy()

This is only used in intel_display() and shouldn't be needed there.
We don't want to keep converting from pll id to pll type so just remove
the function.

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/20190309035727.25389-6-lucas.demarchi@intel.com