drm/nv50: add more 0x100c80 flushy magic

Fixes the !vbo_fifo path in the 3D driver on certain chipsets.  Still not
really any good idea of what exactly the magic achieves, but it makes
things work.

While we're at it, in the PCIEGART path, flush on unbinding also.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>