drm/i915: Configure the TV sense state correctly on GM45 to make TV detection reliable

The TV detection logic is not reliable on the Cantiga platform.
Sometimes the TV will be misdetected as the following two cases:
- TV is misdetected on some laptops. e.g. There is no TV connector
port or no TV is attached. But the TV is shown as connected.
- TV connector type is misdetected. e.g. the component TV is
attached, but the TV is shown as S-video type.

According to the hardware requirement, the TV sense state bits of TV DAC
register should be cleared to zero on Cantiga platfrom.

https://bugzilla.kernel.org/show_bug.cgi?id=14792

Cc: Stable Team <stable@kernel.org>
Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Tested-by: Santi <santi@agolina.net>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>