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

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

Spec shows voltage level 0 as 307.2, 312, or lower and suggests to use
range checks. Prepare for having other frequencies in these ranges by
not comparing the exact frequency.

v2: invert checks by comparing biggest cdclk first (suggested by Ville)

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/20190610214819.9703-1-lucas.demarchi@intel.com