drm/i915/cnl: simplify cnl_procmon_values handling

Make it a little less magical and a little simpler and more hardcoded
so we don't end up with an array that's composed mostly of empty
entries.

v2: Add an enum for the voltage+register values (Ville).

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170822000356.17330-1-rodrigo.vivi@intel.com