drm/nouveau/ttm: avoid using nouveau_drm.ttm.type_vram prior to nv50

Pre-NV50 chipsets don't currently use the MMU subsystem that later
chipsets use, and type_vram is negative here, leading to an OOB memory
access.

This was previously guarded by a chipset check, restore that.

Reported-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 5839172f

 ("drm/nouveau: explicitly specify caching to use")
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>