drm/i915: clean up and clarify audio related register defines

Make audio related register defines conform to existing style: Add _MASK
where relevant, indent the defines for register contents, don't indent
the defines for register addresses, prefix pipe specific register
address defines with underscores, drop self explanatory comments.

No functional changes.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>