drm/i915/audio: Read ELD buffer size from hardware

We currently read the ELD buffer size from hardware on g4x,
but on ilk+ we just hardcode it to 84 bytes. Let's unify
this and just do the hardware readout on all platforms,
in case the size changes in the future or something.

TODO: should perhaps do the readout during driver init and
stash the results somewhere so that we could check that the
connector's ELD actually fits and not even try to enable audio
in that case...

v2: Document the size is in dwords (Jani)

Cc: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Cc: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Cc: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
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/20221026170150.2654-10-ville.syrjala@linux.intel.com