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>

drivers/gpu/drm/mgag200/mgag200_drv.h

@@ -280,7 +280,7 @@ static inline int mgag200_bo_reserve(struct mgag200_bo *bo, bool no_wait)
 {
 	int ret;
 
-	ret = ttm_bo_reserve(&bo->bo, true, no_wait, false, 0);
+	ret = ttm_bo_reserve(&bo->bo, true, no_wait, false, NULL);
 	if (ret) {
 		if (ret != -ERESTARTSYS && ret != -EBUSY)
 			DRM_ERROR("reserve failed %p\n", bo);