The driver unload hangs because the page retirement
kthread cannot be stopped as it is sleeping and waiting
on page retirement event to occur. Add kthread_should_stop()
to the event condition to wake up the kthread when kthread
stop is called during driver unload.

Fixes: 3fdcd0a3 ("drm/amdgpu: Prepare for asynchronous processing of umc page retirement")
Signed-off-by: Mukul Joshi <mukul.joshi@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>