platform-drivers-x86: set backlight type to BACKLIGHT_PLATFORM

Some newly added drivers do not set backlight type, as a result
/sys/class/backlight/<backlight>/type shows incorrect backlight type.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Cc: Corentin Chary <corentin.chary@gmail.com>
Cc: Yin Kangkai <kangkai.yin@intel.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>