nouveau: replace vtxbuf/idxbuf caps with BO_ flags in nouveau_screen and fix uncached...

nouveau: replace vtxbuf/idxbuf caps with BO_ flags in nouveau_screen and fix uncached reads on nv3x

Faster, simpler and more flexible.

Also, we set those flags properly on nv3x so that we don't allocate buffers in GART.

Since on AGP GART is uncached, OpenGL doesn't distinguish between vertex and index buffers, and we don't support hardware index buffers for now, this caused uncached reads.

Also check bind and not usage for PIPE_BIND_* flags, got broken in the gallium-resources transition.