drm/i915: s/freq/cdclk/

Rename the generic sounding freq/frequency parameters to the cdclk
functions to 'cdclk' so that we'll know which clock we're talking about
once we have to deal with the vco frequencies as well.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1462995892-32416-11-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>