drm/i915: Set guardband clipping workaround bit in the right register.

A previous patch, namely:

commit bf97b276
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed Apr 11 20:42:41 2012 +0200

    drm/i915: implement w/a for incorrect guarband clipping

accidentally set bit 5 in 3D_CHICKEN, which has nothing to do with
clipping.  This patch changes it to be set in 3D_CHICKEN3, where it
belongs.

The game "Dante" demonstrates random clipping issues when guardband
clipping is enabled and bit 5 of 3D_CHICKEN3 isn't set.  So the
workaround is actually necessary.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Oliver McFadden <oliver.mcfadden@linux.intel.com>
Acked-by: Paul Menzel <paulepanter@users.sourceforge.net>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>