backend-drm: Don't leak drm_device on shutdown

This was introduced in a partial MR, where the later commits in the new
multi-GPU MR fully fix it, but the initially cherry-picked ones don't.

Signed-off-by: Daniel Stone <daniels@collabora.com>

libweston/backend-drm/drm.c

@@ -2681,6 +2681,7 @@ drm_destroy(struct weston_compositor *ec)
 	weston_launcher_destroy(ec->launcher);
 
 	free(device->drm.filename);
+	free(device);
 	free(b);
 }