backend-drm: Do not create a head for invalid blacklight values

This most likely is a driver fail, as we shouldn't be getting a zero
value for max_brightness. Later on, this will be used to compute a
normalized brightness which would trip a with a division by zero.
Rather than fixing that up, just don't enable backlight support and
reportat that to the user that we have an invalid max_brightness value
and not backlight support.

Fixes: #878

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>