drm/i915: Add the brightness property for SDVO-LVDS

When the sdvo device is detected as SDVO-LVDS, we will check whether the
brightness is supported by issue SDVO enhancement command.
If it is supported, we will add the brightness property and then brightness
can be adjusted.

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>