drm/edid: use the new displayid iterator for finding CEA extension

Neatly reduce displayid boilerplate in code. No functional changes.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/e50f876cecbfee369da887ad19350eee0d89b87f.1617024940.git.jani.nikula@intel.com