ttm: use NULL instead of 0 for ttm_bo_reserve()'s pointer arg.

Fix a sparse warning: ttm_bo_reserve()'s last argument is a
pointer to a struct, so use NULL as nullpointer.

Signed-off-by: Martin Kepplinger <martink@posteo.de>
Reviewed-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>