drm/radeon: disable native backlight control on pre-r6xx asics (v2)

Just use the acpi interface.  That's what windows uses on this
generation and it's the only thing that seems to work reliably
on these generation parts.

You can still force the native backlight interface by setting
radeon.backlight=1

Bug:
https://bugzilla.kernel.org/show_bug.cgi?id=88501



v2: merge into above if/else block

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org