drm/nouveau: Fix a crash at card takedown for NV40 and older cards

NV40 and older cards (pre NV50) reserve a vram bo for the vga memory at
card init. This bo is then freed at card shutdown.  The problem is that
the ttm bo vram manager was already freed. So a crash occurs when the
vga bo is freed. The fix is to free the vga bo prior to freeing the ttm
bo vram manager. There might be other solutions but this seemed the
simplest to me.

Signed-off-by: Jimmy Rentz <jb17bsome@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>