video: s3c-fb: Don't keep device runtime active when open

Allow the controller to be runtime suspended when the screen is blanked
by not taking a runtime reference while the device is open. This allows
greater system wide power savings when used with a standard application
layer and ensures that the screen does not blank unless requested.

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>