drm/radeon: enable native backlight control on old macs

commit 7a26f9ad1b5badfd0200ce2262ad696e2a6b7fbb upstream.

Commit b7bc596e ("drm/radeon: disable native
backlight control on pre-r6xx asics (v2)") accidently
broke backlight control on old mac laptops that use the
on-GPU backlight controller.

Signed-off-by: Nathan-J. Hirschauer <nathanhi@deepserve.info>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>