radv: lower barycentric_at_sample in NIR
More preliminary work for GPL because in some situations the number of samples isn't known at compile time and it will be passed through a new user SGPR.
It was actually a Zink bug because it's required to set the MSAA state when sample shading is used ("If the pipeline is being created with fragment shader state with a fragment shader that either enables sample shading or decorates any variable in the Input storage class with Sample, then pMultisampleState must not be NULL")
I think this MR is still fine though.