drm/amdkfd: Add per-process IDR for buffer handles

Also used for cleaning up on process termination.

v2: Refactored cleanup on process termination

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>