drm/i915: Cache current cdclk frequency in dev_priv

Rather that extracting the current cdclk freuqncy every time someone
wants to know it, cache the current value and use that. VLV/CHV already
stored a cached value there so just expand that to cover all platforms.

v2: Rebased to the latest
v3: Rebased to the latest
v4: Rebased to the latest
v5: Removed spurious call to 'intel_update_cdclk(dev)' based on
    Damien Lespiau's comment

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>