freedreno/ir3: flip texcoord cap

So the last remaining problem before enabling shader-cache turned out to be that in some cases, when shader-cache is in the picture, st_nir_fixup_varying_slots() can be called twice. (I believe once before serializing to cache, and once after?)

But we can choose to just not have this problem, by flipping the texcoord cap and getting rid of the varying slot remapping.

This has the nice side-benefit that (on a6xx), gl_PointCoord is no longer triggering an emit-time slowpath.