drm/i915/regs: split out intel audio register definitions

Split out audio registers to a header of its own to reduce the size of
i915_reg.h.

TODO: Remove direct audio register access from intel_ddi.c. However,
unification of audio get config is cumbersome due to the audio enable
bit being in the DP or HDMI registers on older platforms.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220602094542.1386151-1-jani.nikula@intel.com