mesa/st: Enable scissor test when clearing by quad

The scissor state -is- setup, but the scissor test is not enabled. This can prevent certain optimizations from occurring on tilers where unaffected tiles are thrown out entirely.

I can confirm things still seem to work (and I see a performance improvement on sway), though I'm eagerly awaiting CI since I'm not convinced this is valid.

[vc4/v3d tagged as they should benefit for the same Panfrost will, due to the draw_min/max_x/y tracking]