video: s3c-fb: Take a runtime PM reference when unblanked

When the framebuffer is unblanked hold a runtime PM reference. This
prevents us powering down when userspace has left an image on the
framebuffer and prepares the way for being able to power down the hardware
when an application still has the device open.

Since we now hold a runtime PM reference whenever the display is unblanked
there is no need for the runtime power management to disable and enable
the display, and doing so would lead to runtime PM trying to recurse into
itself when called from the blanking code, so split the runtime PM into
separate functions which only deal with the clocks.  The PM core will
runtime resume the device prior to system suspend.

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>