drm/i915: rename audio ELD registers

Change the definitions from GEN5 to IBX as they aren't in the CPU and
some SNB systems actually shipped with IBX chipsets (or, at least that's
a supported configuration).

The GEN7_* register addresses actually take effect since GEN6 and should
be prefixed by CPT, the PCH code name.

Suggested-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Keith Packard <keithp@keithp.com>