drm: use "0" instead of "" for deprecated driver date

libdrm does not like the empty string for driver date. Use "0" instead,
which has been used by virtio previously.
Reported-by: Steven Price <steven.price@arm.com>
Closes: https://lore.kernel.org/r/9d0cff47-308e-4b11-a9f3-4157dc26b6fa@arm.com
Fixes: 7fb8af67 ("drm: deprecate driver date")
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240510090951.3398882-1-jani.nikula@intel.comReviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Tested-by: Steven Price <steven.price@arm.com>
Reviewed-by: Simon Ser <contact@emersion.fr>

drivers/gpu/drm/drm_ioctl.c

@@ -530,9 +530,9 @@ int drm_version(struct drm_device *dev, void *data,
 	err = drm_copy_field(version->name, &version->name_len,
 			dev->driver->name);
 
-	/* Driver date is deprecated. Return the empty string. */
+	/* Driver date is deprecated. Userspace expects a non-empty string. */
 	if (!err)
-		err = drm_copy_field(version->date, &version->date_len, "");
+		err = drm_copy_field(version->date, &version->date_len, "0");
 	if (!err)
 		err = drm_copy_field(version->desc, &version->desc_len,
 				dev->driver->desc);