drm/amdgpu: free handles after fini the context

This will make sure all the submissions from different contexts gets
finished, and then we close the session and free up the handles.

This will fix the issue that session clean-up is not get done properly,
when with the command `kill -9'

Signed-off-by: Leo Liu <leo.liu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>