drm/i915: Add the enhancement property for SDVO-TV

When the sdvo device is detected as SDVO-TV, we will check whether the
sepecific picture enhancement is supported. If it is supported, we will
add the corresponnding property for SDVO-TV. We will add the following
property for the SDVO-TV enhancements if they are supported:
 * Contrast/Brightness/Saturation/Hue.
 * left/right/top/bottom margin: This is implemented by using the
   horizontal/vertical overscan enhancements. When the overscan
   enhancements are supported, the above properties will be added. This is
   to be compatible with what we have done in integrated-TV.
 * horizontal pos/vertical pos.

http://bugs.freedesktop.org/show_bug.cgi?id=22891

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