drm/i915: Add tracking for CDCLK bypass frequency

The CDCLK bypass frequency can vary on upcoming platforms, so prepare
for that now by tracking its value in the CDCLK state.

Currently on BDW+ the bypass frequency is always the reference clock and
I didn't bother with earlier platforms since it's not all that clear
what's the bypass clock on those.

I also didn't bother adding support for changing this frequency, since
atm I don't see any need for it.

Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180117172508.15993-1-imre.deak@intel.com