nv30: set usage to staging so that the buffer is allocated in GART

nv30: set usage to staging so that the buffer is allocated in GART

The code a few lines below expects to migrate the bo in question to
VRAM. Since we're filling the initial data via CPU, it's more efficient
to create the temporary buffer in GART. There is no "push" method
implemented, otherwise we'd use that instead.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: mesa-stable@lists.freedesktop.org