drm/i915: Store cdclk PLL reference clock under dev_priv

Future platforms will have multiple options for the cdclk PLL reference
clock, so let's start tracking that under dev_priv alreday on SKL,
although on SKL it's always 24 MHz.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463172100-24715-15-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>