drm/ttm: Fix a few sparse warnings

The final parameter to ttm_bo_reserve() is a pointer, therefore callers
should use NULL instead of 0.

Fixes a bunch of sparse warnings of this type:

	warning: Using plain integer as NULL pointer

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/qxl/qxl_object.h b/drivers/gpu/drm/qxl/qxl_object.h
index d458a14..83a4232 100644
--- a/drivers/gpu/drm/qxl/qxl_object.h
+++ b/drivers/gpu/drm/qxl/qxl_object.h
@@ -31,7 +31,7 @@
 {
 	int r;
 
-	r = ttm_bo_reserve(&bo->tbo, true, no_wait, false, 0);
+	r = ttm_bo_reserve(&bo->tbo, true, no_wait, false, NULL);
 	if (unlikely(r != 0)) {
 		if (r != -ERESTARTSYS) {
 			struct qxl_device *qdev = (struct qxl_device *)bo->gem_base.dev->dev_private;
@@ -67,7 +67,7 @@
 {
 	int r;
 
-	r = ttm_bo_reserve(&bo->tbo, true, no_wait, false, 0);
+	r = ttm_bo_reserve(&bo->tbo, true, no_wait, false, NULL);
 	if (unlikely(r != 0)) {
 		if (r != -ERESTARTSYS) {
 			struct qxl_device *qdev = (struct qxl_device *)bo->gem_base.dev->dev_private;