drm/i915/backlight: handle enabled but zero duty cycle at module load

Don't consider enabled but zero duty cycle backlight disabled. Clamp
level between min and max for sanity.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471939811-9817-1-git-send-email-maarten.lankhorst@linux.intel.com


Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>