drm/radeon: drop confusing message about backlight control

The message is harmless and confusing.  On PX systems,
there is one ATIF method, but potentially multiple GPUs
leading to an error on the GPU with no backlight control.

Bug: https://bugzilla.kernel.org/show_bug.cgi?id=115011


Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Hans de Goede <jwrdegoede@fedoraproject.org>