gallium: consolidate bypass_vs and bypass_clipping flags

gallium: consolidate bypass_vs and bypass_clipping flags

The draw module provides a similar interface to the driver which
is retained as various bits of hardware may be able to take on
incremental parts of the vertex pipeline.  However, there's no
need to advertise all this complexity to the state tracker.

There are basically two modes now - normal and passthrough/screen-coords.