drm/amdgpu: Drop CONFIG_BACKLIGHT_CLASS_DEVICE ifdefs

The DRM_AMDGPU Kconfig code contains:

	select BACKLIGHT_CLASS_DEVICE

So the condition these ifdefs test for is always true, drop them.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>