drm/edid: add drm_edid helper for drm_edid_to_sad()

We'll need to propagate drm_edid everywhere.

v2: Handle NULL EDID pointer (Ville, CI)

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/14aa1e44caa2e9c1b54c2f2a471d2fd7d3f62d5d.1652097712.git.jani.nikula@intel.com