amdkfd: Fix memory leak on process deregistration

struct device_process_node was allocated during process registration but
not released at process deregistration.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Jay Cornwall <jay.cornwall@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@amd.com>