drm/i915/ringbuffer: Exclude last 2 cachlines of ring on 845g

drm/i915/ringbuffer: Exclude last 2 cachlines of ring on 845g

BugLink: http://bugs.launchpad.net/bugs/987337

commit 27c1cbd06a7620b354cbb363834f3bb8df4f410d upstream.

The 845g shares the errata with i830 whereby executing a command
within 2 cachelines of the end of the ringbuffer may cause a GPU hang.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>