drm/i915: Add calc_voltage_level display vfunc

With all of the cdclk function consolidation, we can cut down on a lot
of platform if/else logic by creating a vfunc that's initialized at
startup.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190910154252.30503-7-matthew.d.roper@intel.com