drm/nv50: let the card handle the initial context switch

Our PFIFO/PGRAPH context save/load functions don't really work well
(at all?) on nv5x yet.  Depending on what random state the card is
in before the drm loads, fbcon probably won't work correctly.

Luckily we've setup the GPU in such a way that it'll actually do a
hw context switch for the first context.  Not sure of how successful
this'd be currently on the older chips (actually, pretty sure it won't
work), so NV50 only for now.