drm/edid: convert cea_db_iter_edid_begin() to drm_edid

We'll need to propagate drm_edid everywhere.

v2: Handle NULL drm_edid

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/7d9f060815086e1e4ea037dbfb9fd8ad54d1f25e.1652097712.git.jani.nikula@intel.com