drm/radeon: add a backlight quirk for Amilo Xi 2550

Only the acpi backlight seems to work.  Using the
radeon backlight controller causes the backlight to
go off.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=81382



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>