nvfx: add NOUVEAU_VTXIDX_IN_VRAM variable to put vertex/index buffers in VRAM

nvfx: add NOUVEAU_VTXIDX_IN_VRAM variable to put vertex/index buffers in VRAM

On some systems, putting vertex and index buffers in VRAM instead of GART
memory eliminates massive graphics corruption which is otherwise present,
due to unclear causes.

This patch adds an environment variable that does that, along with helpful
messages.

It turns it on by default on G7x, as it is what I am seeing corruption
on and some other reports also seemed to pinpoint these cards.