drm/i915: Renaming CCK related reg definitions

Rename the DISPLAY_TRUNK_* and DISPLAY_FREQUENCY_* bits to CCK_... instead
of DISPLAY_... to make it clear they apply to all CCK clock control registers.
Suggested by Ville.

Signed-off-by: Vandana Kannan <vandana.kannan@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>