Don't bind a PS if the last vertex stage doesn't submit a POS
This replaces !157 (closed)
This fixes piglit glsl-1.50@execution@geometry@gs-point-unused-outputs
The actual patches of this MR are the last two of which first patch is kind of a cleanup patch that mase a lot of sense in !157 (closed), but it doesn't hurt to get this added, so I kept it.
To run the piglit one also needs to enable PIPE_SHADER_CAP_INDIRECT_CONST_ADDR
Edited by Gert Wollny