drm/i915/sdvo: Do ELD hardware readout

Read out the ELD from the hw so the state checker can verify it.

v2: Check the "ELD valid" bit separately
v3: Fix ELD tx rate handling during readout

Cc: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Cc: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Jani Nikula <jani.nikula@intel.com> #v1
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/20230124144628.4649-10-ville.syrjala@linux.intel.com