intel/fs: Make per-sample and coarse dispatch tri-state

intel/fs: Make per-sample and coarse dispatch tri-state

Whenever one of them is BRW_SOMETIMES, we depend on dynamic flag pushed
in as a push constant.  In this case, we have to often have to do the
calculation both ways and SEL the result.  It's a bit more code but
decouples MSAA from the shader key.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21094>