drm/nouveau: Fix out-of-bounds access when deferencing MMU type

The value of struct drm_device.ttm.type_vram can become -1 for unknown
types of memory (see nouveau_ttm_init()). This leads to an out-of-bounds
error when accessing struct nvif_mmu.type[]:

  [   18.304116] ==================================================================
  [   18.311649] BUG: KASAN: slab-out-of-bounds in nouveau_ttm_io_mem_reserve+0x17a/0x7e0 [nouveau]
  [   18.320415] Read of size 1 at addr ffff88810ffac1fe by task systemd-udevd/342
  [   18.327681]
  [   18.329208] CPU: 1 PID: 342 Comm: systemd-udevd Tainted: G            E     5.10.0-rc2-1-default+ #581
  [   18.338681] Hardware name: Dell Inc. OptiPlex 9020/0N4YC8, BIOS A24 10/24/2018
  [   18.346032] Call Trace:
  [   18.348536]  dump_stack+0xae/0xe5
  [   18.351919]  print_address_description.constprop.0+0x17/0xf0
  [   18.357787]  ? nouveau_ttm_io_mem_reserve+0x17a/0x7e0 [nouveau]
  [   18.363818]  __kasan_report.cold+0x20/0x38
  [   18.368099]  ? nouveau_ttm_io_mem_reserve+0x17a/0x7e0 [nouveau]
  [   18.374133]  kasan_report+0x3a/0x50
  [   18.377789]  nouveau_ttm_io_mem_reserve+0x17a/0x7e0 [nouveau]
  <...>
  [   18.767690] Allocated by task 342:
  [   18.773087]  kasan_save_stack+0x1b/0x40
  [   18.778890]  __kasan_kmalloc.constprop.0+0xbf/0xd0
  [   18.785646]  __kmalloc_track_caller+0x1be/0x390
  [   18.792165]  kstrdup_const+0x46/0x70
  [   18.797686]  kobject_set_name_vargs+0x2f/0xb0
  [   18.803992]  kobject_init_and_add+0x9d/0xf0
  [   18.810117]  ttm_mem_global_init+0x12c/0x210 [ttm]
  [   18.816853]  ttm_bo_global_init+0x4a/0x160 [ttm]
  [   18.823420]  ttm_bo_device_init+0x39/0x220 [ttm]
  [   18.830046]  nouveau_ttm_init+0x2c3/0x830 [nouveau]
  [   18.836929]  nouveau_drm_device_init+0x1b4/0x3f0 [nouveau]
  <...>
  [   19.105336] ==================================================================

Fix this error, by not using type_vram as an index if it's negative.
Assume default values instead.

The error was seen on Nvidia G72 hardware.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Acked-by: Christian König <christian.koenig@amd.com>
Fixes: 1cf65c45 ("drm/ttm: add caching state to ttm_bus_placement")
Cc: Christian König <christian.koenig@amd.com>
Cc: Michael J. Ruhl <michael.j.ruhl@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: VMware Graphics <linux-graphics-maintainer@vmware.com>
Cc: Roland Scheidegger <sroland@vmware.com>
Cc: Huang Rui <ray.huang@amd.com>
Cc: Felix Kuehling <Felix.Kuehling@amd.com>
Cc: Hawking Zhang <Hawking.Zhang@amd.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Likun Gao <Likun.Gao@amd.com>
Cc: dri-devel@lists.freedesktop.org
Cc: nouveau@lists.freedesktop.org
Cc: virtualization@lists.linux-foundation.org
Cc: spice-devel@lists.freedesktop.org
Cc: amd-gfx@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20201110133655.13174-1-tzimmermann@suse.de