drm/i915: Update cached cdclk state from broxton_init_cdclk()

Let's make sure our cached cdclk state is accurate right after
broxton_init_cdclk() whether or not we end up changing the cdclk
frequency.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463172100-24715-18-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>