drm/gma500: Change registered backlight device type to raw/native

Change the type for the registered backlight class device from platform
to raw/native.

The poulsbo/cedarview/oaktrail backlight support is using native GPU
backlight control and as such the type should be raw (aka native) as
is done by all the other native GPU backlight driver code.

Note this will not change much from userspace's point of view.
poulsbo/cedarview laptops typically offer both an ACPI-video
backlight interface as well as the native GPU backlight interface.

The /sys/class/backlight/acpi_video0 has a type of firmware and
userspace typically looks for firmware devices before looking
for platform devices. The typical standard lookup order is:
firmware -> platform -> raw

This means that both before and after this change typical userspace
backlight consumers (sich as e.g. GNOME) will prefer the firmware
acpi_video0 backlight device.

This has been tested on a Packard Bell Dot SC (Intel Atom N2600, cedarview)
and a Sony Vaio vpc-x11s1e (Intel N540, poulsbo) laptop.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220917205920.647212-3-hdegoede@redhat.com