Fixes a PT_NOT_PRESENT error cause by:
- allocating in VRAM
- emitting GART relocs to 0x17bc/0x17c0, moving the buffer
- telling the bufmgr that the buffer should be in VRAM when we use it,
  but not correcting the value sent to 0x17bc/0x17c0.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>