drm/i915: Start tracking voltage level in the cdclk state

For CNL we'll need to start considering the port clocks when we select
the voltage level for the system agent. To that end start tracking the
voltage in the cdclk state (since that already has to adjust it).

v2: s/voltage/voltage_level/ (Rodrigo)

Cc: Mika Kahola <mika.kahola@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171024095216.1638-3-ville.syrjala@linux.intel.com