drm/edid: use a temp variable for sads to drop one level of dereferences

Use a temporary variable struct cea_sad *, instead of using struct
cea_sad ** directly with the double dereferences. It's arguably easier
on the eyes, and drops a set of parenthesis too.

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/b6e2f295ae5491c2bb0f528508f0f5fca921dc77.1698747331.git.jani.nikula@intel.com