video: exynos_dp: use devm_ functions

The devm_ functions allocate memory that is released when a driver
detaches. This makes the code smaller and a bit simpler.

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