drm/tinydrm: helpers: Properly fix backlight dependency

BACKLIGHT_CLASS_DEVICE was selected in the last version of the
tinydrm patchset to fix the backlight dependency, but the
ifdef CONFIG_BACKLIGHT_CLASS_DEVICE was forgotten. Fix that.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>