r300: skip draws instead of using dummy vertex shader

Drawing more than ~30 points with dummy shader under X leads to hangs with RV370. Adding few more instructions to the dummy shader works around. This fixes hanging piglit glsl-vs-raytrace-bug26691

I have no idea what the underlying problem is, however this approach seems quite safe (we don't care what we use for a dummy shader anyway) and the benefit of being able to run testsuites without hangs should outweigh the downsides. Also since there are no R3xx docs and I was not able to find any clue in R5xx docs, I'm not sure if there is a better way.

EDIT: New version skips the draws altogether as Anholt suggested.