vc4: Disable non-uniform loops in the vertex shader

[anholt]

piglit glsl-vs-loop and some DEQP tests GPU hang because the non-dispatched vertex shader channels never trigger their break condition. We should emit a GLSL link failure for non-uniform loops, to fix the DEQP conformance issue and avoid the hangs. This will involve adding a new PIPE_SHADER_CAP_* to gallium, and using that in st_glsl_to_nir.c to check for non-uniform loops and emit the error.

This will probably want Jason's pass