nouveau: make sure initial kalloc for user bo ends up in the right place

- Currently reloc'ing a user bo to gart will first cause an allocation in vram,
which is then written to by cpu, then the bo gets moved to gart.

Acked-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Maarten Maathuis <madman2003@gmail.com>