freedreno/a6xx: fix max-scissor opt
On a6xx we need a 0,0 based scissor in the binning pass, but can use the blit-scissor to avoid restore/resolve of untouched pixels, and use the conditional execution if the IB to bin to skip bins with no geometry (due to the scissor).
Signed-off-by: Rob Clark robdclark@chromium.org