drm/amdgpu: Fix module unload hang with RAS enabled

drm/amdgpu: Fix module unload hang with RAS enabled

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: 3fdcd0a31d7a ("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>