drm/i915: Disable FBC on 915GM and 945GM.

It is causing hangs after a suspend/resume cycle with the default
powersave=1 module option on these chipsets since 2.6.32-rc.

BugLink: http://bugs.launchpad.net/bugs/492392


Signed-off-by: Robert Hooker <sarvatt@ubuntu.com>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>