drm/i915: Use cdclk_state->voltage on VLV/CHV

Store the punit DSPFREQUAR value into cdclk_state->voltage on
VLV/CHV. Since we can actually read that out from the hardware
this can give us a bit more cross checking between the hardware
and software state.

v2: Don't break waiting for cdclk change on VLV/CHV
v3: Split out the cdclk sanity check in vlv_set_cdclk() (Rodrigo)
v4: 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/20171024095216.1638-4-ville.syrjala@linux.intel.com