drm/edid: add helpers to get/set struct cea_sad from/to 3-byte sad

Add helpers to pack/unpack SADs. Both ways and non-static, as follow-up
work needs them.

v2: Add include to get the declarations

Cc: Mitul Golani <mitulkumar.ajitkumar.golani@intel.com>
Reviewed-by: Mitul Golani <mitulkumar.ajitkumar.golani@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/21d657ca854ce26423b461c0bb71e7a0727ba437.1698747331.git.jani.nikula@intel.com