drm/i915/skl: use ranges for voltage level lookup

Like was done for ICL, let's convert the voltage level lookup to use
frequency ranges rather than individual frequencies. For deciding the
voltage, the individual value doesn't really matter.

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/20190610214847.9865-1-lucas.demarchi@intel.com