radv: Slight changes to NGG culling determination.

The original limits were chosen very conservatively because at that time we didn't have a good understanding on the perf impact of shader culling. We also (mistakenly) forgot to update the code for GFX11 so it had the same limits as GFX10 and made culling very limited.

Since then, we've seen some use cases that have a higher amount of PS params but still benefit from shader culling on GFX10.3+.

Note that further testing is required to determine whether it should be enabled by default on GFX11+ which we still haven't done yet.

Also, the LLVM backend is now capable of emitting correct code for the culling shaders so we don't need to discriminate it anymore.