drm/amd/display: Drop CONFIG_BACKLIGHT_CLASS_DEVICE ifdefs

Remove CONFI_BACKLIGHT_CLASS_DEVICE ifdef
that was accidently introduced back.

Reviewed-by: Hamza Mahfooz <Hamza.Mahfooz@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>