video: s3c-fb: Unify runtime and system PM functions

The s3c-fb driver has separate runtime and system PM functions but the
implementations are identical so far as I can tell so unify them for
simplicity.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>