video: s3c-fb: use devm_clk_get()

The devm_ functions allocate memory that is released when a driver
detaches. This patch uses devm_clk_get() for these functions.

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