drm/i915: move and split audio under display.audio and display.funcs

Move display audio related members under drm_i915_private display
sub-struct.

Split audio funcs to display.funcs to follow the same pattern as all the
other display functions.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/51c0b966963a8b47efe3a431e0ce106bdd5269c5.1661346845.git.jani.nikula@intel.com