video: s3c-fb: add video clock running at data under-flow

This patch adds video clock running at data under-flow.
It means that the pixel clock is continuously provided
to lcd module, even when under-run occurs.

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