drm/i915: Perform a central cdclk state sanity check

drm/i915: Perform a central cdclk state sanity check

WARN if the cdclk state doesn't match what we expect after programming.
And let's remove the WARN from bdw_set_cdclk() that's trying to achieve
the same thing in a more limite fashion.

Also take the opportunity to refactor the code to use a common function
for dumping out a cdclk state.

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>
Link: https://patchwork.freedesktop.org/patch/msgid/20171024095216.1638-11-ville.syrjala@linux.intel.com
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>