drm/amdkfd: fix amdkfd use-after-free GP fault

Fix GP fault caused by dev_info() reference to a struct device*
after the device has been freed (use after free).
kfd_chardev_exit() frees the device so 'kfd_device' should not
be used after calling kfd_chardev_exit().

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>