drm/i915: Use cdclk_state->voltage on BDW

Track the system agent voltage we request from pcode in the cdclk state
on BDW. Annoyingly we can't actually read out the current value since
there's no pcode command to do that, so we'll have to just assume that
it worked.

v2: Keep the WARN_ON (Rodrigo)
v3: 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/20171024095216.1638-5-ville.syrjala@linux.intel.com