gpu: Add Intel GMA500(Poulsbo) Stub Driver

Currently, there have no GMA500(Poulsbo) native video driver to support
intel opregion. So, use this stub driver to enable the acpi backlight
control sysfs entry files by requrest acpi_video_register.

[airlied: fix warnings]

Signed-off-by: Lee, Chun-Yi <jlee@novell.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>