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