radv: fix alpha-to-coverage with alpha-to-one when MRTZ is also exported
alpha-to-coverage with alpha-to-one is just broken on RADV.
This fixes it when depth, stencil or samplemask are also exported. The non-MRTZ case will be fixed in a separate MR because it's going to be even more intrusive.
This might fix #12217 (closed) if the shader exports depth/stencil/samplemask.
Given the fixes aren't small, I think I will not backport them and I will probably add a new drirc to disable alpha-to-one in the mean time.