drm/i915: Don't pass dev_priv to broxton_calc_cdclk()

broxton_calc_cdclk() doesn't need dev_priv for anything, so let's not
bother passing it around.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1462995892-32416-5-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>