drm/i915: fixup pageflip ringbuffer commands for i8xx

Add a new path for 2nd gen chips that uses the commands for i81x
chips (where public docs do exist) augmented with the plane bits
from i915. It seems to work and doesn't result in a black screen
like before.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: stable@kernel.org
[anholt: resolved against conflict]
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>