drm/edid: clean up edid_is_zero()

Simplify, rename, take void pointer. No need for the drm_ prefix for
internal helpers.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/15220580f07b63c92a3e448347cf59b297990407.1648752228.git.jani.nikula@intel.com