nouveau: Nuke DMA_OBJECT_INIT ioctl (bumps interface to 0.0.7)

For various reasons, this ioctl was a bad idea.

At channel creation we now automatically create DMA objects covering
available VRAM and GART memory, where the client used to do this themselves.

However, there is still a need to be able to create DMA objects pointing at
specific areas of memory (ie. notifiers).  Each channel is now allocated a
small amount of memory from which a client can suballocate things (such as
notifiers), and have a DMA object created which covers the suballocated area.
The NOTIFIER_ALLOC ioctl exposes this functionality.