drm/eld: add helpers to modify the SADs of an ELD

Occasionally it's necessary for drivers to modify the SADs of an ELD,
but it's not so cool to have drivers poke at the ELD buffer directly.

Using the helpers to translate between 3-byte SAD and struct cea_sad,
add ELD helpers to get/set the SADs from/to an ELD.

v2: s/i/sad_index/ (Mitul)

Cc: Mitul Golani <mitulkumar.ajitkumar.golani@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Mitul Golani <mitulkumar.ajitkumar.golani@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8e9a05f2b1e0dd184132d636e1e778e8917ec25d.1698747331.git.jani.nikula@intel.com