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>
diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_chardev.c b/drivers/gpu/drm/amd/amdkfd/kfd_chardev.c
index cc61ec2..62c3d9c 100644
--- a/drivers/gpu/drm/amd/amdkfd/kfd_chardev.c
+++ b/drivers/gpu/drm/amd/amdkfd/kfd_chardev.c
@@ -526,7 +526,7 @@ static int kfd_ioctl_dbg_unregister(struct file *filep,
 	long status;
 
 	dev = kfd_device_by_id(args->gpu_id);
-	if (!dev)
+	if (!dev || !dev->dbgmgr)
 		return -EINVAL;
 
 	if (dev->device_info->asic_family == CHIP_CARRIZO) {