i965: Combine the Gen6 SF and Clip viewport atoms.

i965: Combine the Gen6 SF and Clip viewport atoms.

The next patch will make the guardband calculation dependent on the
transformation matrix.  Instead of computing it in both atoms, just
combine them into a single atom.

Cc: "17.0" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>