nouveau: rewrite gpu object code

Allows multiple references to a single object, needed to support PCI(E)GART
scatter-gather DMA objects which would quickly fill PRAMIN if each channel
had its own.

Handle per-channel private instmem areas.  This is needed to support NV50,
but might be something we want to do on earlier chipsets at some point?

Everything that touches PRAMIN is a GPU object.