drm/i915: Read czclk from CCK on vlv/chv

As with the cdclk, read out czclk from CCK as well. This gives us the
real current value and avoids having to decode fuses and whatnot.

Also store it in kHz under dev_priv like we do for cdlck since it's not
just an rps related clock, and having it in kHz is more
standard/convenient for some things.

Imre also pointed out that we currently fail to read czclk on VLV, which
means the PFI credit programming isn't working as expected.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>