drm/i915/bxt: Sanitize the DBUF HW state together with CDCLK

When determining whether CDCLK is enabled by BIOS and so we should skip
reprogramming it, we didn't check the related DBUF power request and
state. In theory BIOS could enable one without the other so check for
this case and reprogram things if something is amiss.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459515767-29228-13-git-send-email-imre.deak@intel.com