drm/i915/audio: Clean up the PCH type checks

Use explicit PCH type checks to make it more clear
which platforms use which codepaths.

Also reorder the branches in ibx_audio_regs_init()
a bit to be more in chronological order.

Cc: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Cc: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230124144628.4649-14-ville.syrjala@linux.intel.com