drm/amdkfd: destroy dbgmgr in notifier release

amdkfd need to destroy the debug manager in case amdkfd's notifier
function is called before the unbind function, because in that case,
the unbind function will exit without destroying debug manager.

Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
CC: Stable <stable@vger.kernel.org>