i965: Scissor to the viewport when rendering points/lines.

i965: Scissor to the viewport when rendering points/lines.

We're about to start allowing wide points/lines whose vertices are
outside the viewport past the clipper.  This scissoring hack ensures
that any fragments generated are still restricted to the viewport.

It is not necessary on Gen8+ as those platforms already discard
fragments which are outside the viewport.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94453
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94454
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>