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: NThierry Reding <treding@nvidia.com>
Reviewed-by: NAlex Deucher <alexander.deucher@amd.com>
Signed-off-by: NDave Airlie <airlied@redhat.com>