drm/amdkfd: CRIU export dmabuf handles for GTT BOs

Export dmabuf handles for GTT BOs so that their contents can be accessed
using SDMA during checkpoint/restore.

v2: Squash in fix from David to set dmabuf handle to invalid for BOs
that cannot be accessed using SDMA during checkpoint/restore.

Signed-off-by: David Yat Sin <david.yatsin@amd.com>
Reviewed-by : Rajneesh Bhardwaj <rajneesh.bhardwaj@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>