We don't need to support all the color buffers for advanced blend, just
cb0. For Fermi, we use the special binding slots so that we don't
overlap with user textures, while Kepler+ gets a dedicated position for
the fb handle in the driver constbuf.

This logic is only triggered when a FBFETCH is actually present so it
should be a no-op most of the time.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>