Potential fix for doom3 lockups. Seems that there is a conflict

Potential fix for doom3 lockups.  Seems that there is a conflict
between the vertex cache, the vertex shader and the clipping stages,
all of which are competitors for URB entries assigned to the VS unit.
This change reduces the maximum number of clip and VS threads by
enough to ensure that they cannot consume all the available URB
entries, and then reduces the number somewhat more up to an arbitary
amount I discovered by trial and error.  Unfortunately trial and error
solutions don't inspire total confidence...