drm/i915: Convert the ddi cdclk code to get_display_clock_speed

Unify the HSW/BDW/SKL cdclk extraction code to conform to the same
.get_display_clock_speed() mold that all the other platforms
use.

v2: Update due to SKL code getting added

v3: Rebase on top of -nightly (introduction of intel_audio.c) (Mika Kahola)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Add v3 note as suggested by Damien.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>