drm/i915: Use cached cdclk value

drm/i915: Use cached cdclk value

Rather than reading out the current cdclk value use the cached value we
have tucked away in dev_priv.

v2: Rebased to the latest
v3: Rebased to the latest
v4: Fix for patch style problems

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>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>