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/vmwgfx/vmwgfx_drv.c b/drivers/gpu/drm/vmwgfx/vmwgfx_drv.c
index f31a754..18b54aca 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_drv.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_drv.c
@@ -316,7 +316,7 @@
 	if (unlikely(ret != 0))
 		return ret;
 
-	ret = ttm_bo_reserve(bo, false, true, false, 0);
+	ret = ttm_bo_reserve(bo, false, true, false, NULL);
 	BUG_ON(ret != 0);
 
 	ret = ttm_bo_kmap(bo, 0, 1, &map);