drm/nouveau/dmem: device memory helpers for SVM

Device memory can be use in SVM, in which case we do not have any of
the existing buffer object. This commit add infrastructure to allow
use of device memory without nouveau_bo. Again this is a temporary
solution until a rework of GPU memory management.

Signed-off-by: Jérôme Glisse <jglisse@redhat.com>

drivers/gpu/drm/nouveau/nouveau_drv.h

@@ -212,6 +212,8 @@ struct nouveau_drm {
 	struct dev_pm_domain vga_pm_domain;
 
 	struct nouveau_svm *svm;
+
+	struct nouveau_dmem *dmem;
 };
 
 static inline struct nouveau_drm *