drm/i915: Rename vlv_cdclk_freq to cdclk_freq

Rename vlv_cdclk_freq to cdclk_freq so that it can be used for all
platforms as required. Needed by the next patch.

Signed-off-by: Vandana Kannan <vandana.kannan@intel.com>
Signed-off-by: A.Sunil Kamath <sunil.kamath@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>