drm: do not debug log about missing CEA extensions on NULL edid

Make the drm_edid_to_eld() function useful for resetting, not just
setting, the ELD and HDMI VSDB data, without debug warnings about
missing CEA extensions.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1487344854-18777-3-git-send-email-jani.nikula@intel.com