xf86drm: fix mem leak in drm_usb_dev_path()

`sysfs_uevent_get()` returns a `strndup()`ed string, which must be `free()`d.

Fixes: bf63f8ac ("libdrm: Handle usb_interface devices for usb parsing")
Reviewed-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>