drm/i915: enable low power render writes on GEN3 hardware.

A lot of 945GMs have had stability issues for a long time, this manifested as X hangs, blitter engine hangs, and lots of crashes.

one such report is at:
https://bugs.freedesktop.org/show_bug.cgi?id=20560



along with numerous distro bugzillas.

This only took a week of digging and hair ripping to figure out.

Tracked down and tested on a 945GM Lenovo T60,
previously running
x11perf -copypixwin500
or
x11perf -copywinpix500
repeatedly would cause the GPU to wedge within 4 or 5 tries, with random busy bits set.

After this patch no hangs were observed.

cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>