compositor-drm: Don't warn about missing backlight control

Not every output will have a backlight control, and even if it does we
may just not be able to find it. Not having backlight control isn't an
error, so don't spam the log with it, as doing so can confuse users into
thinking this is an actual error which is responsible for their real
problems.

Signed-off-by: Daniel Stone <daniels@collabora.com>