drm/i915/bxt: Add HW state verification for DDI PHY and CDCLK

I caught a few errors in our current PHY/CDCLK programming by sanity
checking the actual programmed state, so I thought it would be also
useful for the future. In addition to verifying the state after
programming it also verify it after exiting DC5, to make sure DMC
restored/kept intact everything related.

v2:
- Inlining __phy_reg_verify_state() doesn't make sense and also
  incorrect, so don't do it (PW/CI gcc)
v3:
- Rebase on latest -nightly

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: David Weinehall <david.weinehall@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459780030-15781-1-git-send-email-imre.deak@intel.com