drm/i915: switch off backlight for backlight class 0 brightness

Make backlight class sysfs brightness 0 value switch off the backlight
for connectors that have the backlight_power callback defined. For eDP,
this has the similar caveats regarding power savings as bl_power as only
the power sequencer backlight control is switched off.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed_by: Clinton Taylor <Clinton.A.Taylor@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>