radv: Enable NGG culling by default on GFX10.3
I'm not aware of any regressions, and the latest benchmarks seem favorable, so let's enable this by default.
- A new debug flag environment variable RADV_DEBUG=nonggc is added to disable this feature on GPUs where it is enabled by default.
- The previous perf test flag RADV_PERFTEST=nggc will not be needed on GFX10.3 anymore but it can still be used to enable the feature on GPUs where it isn't on by default.