i965/fs: Extend back-end interface for limiting the shader dispatch width.

i965/fs: Extend back-end interface for limiting the shader dispatch width.

This replaces the current fs_visitor::no16() interface with
fs_visitor::limit_dispatch_width(), which takes an additional
parameter allowing the caller to specify the maximum dispatch width a
shader can be compiled with.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>