drm/i915: Use cached cdclk value in i915_audio_component_get_cdclk_freq()

No point in reading the cdclk out from the hardware every single time
since we have it cached already. Just return the cached value to the
audio driver.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461689194-6079-3-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Mika Kahola <mika.kahola@intel.com>