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>

drivers/gpu/drm/nouveau/nouveau_bo.c

@@ -350,14 +350,13 @@ set_placement_list(struct nouveau_drm *drm, struct ttm_place *pl, unsigned *n,
 
 	if (domain & NOUVEAU_GEM_DOMAIN_VRAM) {
 		struct nvif_mmu *mmu = &drm->client.mmu;
-		const u8 type = mmu->type[drm->ttm.type_vram].type;
 
 		pl[*n].mem_type = TTM_PL_VRAM;
 		pl[*n].flags = flags & ~TTM_PL_FLAG_CACHED;
 
 		/* Some BARs do not support being ioremapped WC */
 		if (drm->client.device.info.family >= NV_DEVICE_INFO_V0_TESLA &&
-		    type & NVIF_MEM_UNCACHED)
+		    mmu->type[drm->ttm.type_vram].type & NVIF_MEM_UNCACHED)
 			pl[*n].flags &= ~TTM_PL_FLAG_WC;
 
 		(*n)++;