freedreno/a6xx: Drop struct stage array

This now boils down to just picking between binning or vertex shader
and dummy_fs or real fs, which we can do in a couple of lines of code
instead.  The constlen logic isn't doing what it thinks it's doing,
both constlens at this point

  MAX2(s[VS].constlen, align(state->bs->constlen, 4));

are binning shader constlens.  We'll have to revisit the constlen
logic, but this commit doesn't change how it works.

Reviewed-by: Rob Clark <robdclark@gmail.com>