video: s3c-fb: Hold runtime PM references when touching registers

Take a runtime PM reference whenever updating registers in preparation
for suspending the device when the framebuffer is blanked.

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>