drm/i915: sprite init failure on pre-SNB is not a failure

We can call the plane init function unconditionally, but don't need to
complain if it fails, since that will only happen if we're out of
memory (and other things will fail) or if we're on the wrong platform
(which is ok).

And remove the DRM_ERRORs from the sprite code itself to avoid dmesg
spam.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Keith Packard <keithp@keithp.com>