nak: Match on the SrcRef directly in Src::is_nonzero()

Calling as_u32() isn't really doing us any good here since we want to
fail for SrcRef::Zero anyway.  Also assert that src_mod == None since we
don't handle FNeg in this path.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/34818>
1 file changed