video: s3c-fb: clear SHADOWCON register when clearing hardware window registers

All bits of SHADOWCON register should be cleared when clearing
hardware window registers; however, some bits of SHADOWCON register
are not cleared previously.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>