freedreno/a3xx: binning-pass vertex shader variant

freedreno/a3xx: binning-pass vertex shader variant

Now that we have the infrastructure for shader variants, add support to
generate an optimized shader for hw binning pass (with varyings/outputs
other than position/pointsize removed).  This exposes the possibility
that the shader uses fewer constants than what is bound, so we have to
take care to not emit consts beyond what the shader uses, lest we
provoke the wrath of the HLSQ lockup!

Signed-off-by: Rob Clark <robclark@freedesktop.org>