drm/amdkfd: Add crash protection in debugger register path

After debugger is registered, the pqm_destroy_queue fails because is_debug
is true, the queue should not be removed from process_queue_list since
the count is not reduced.

Test application calls debugger unregister without register debugger, add
null pointer check protection to avoid crash for this case

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>