video: exynos_dp: use devm_clk_get function

The devm_clk_get function allocates memory that is released when a driver
detaches. This patch uses this function for data that is allocated in the probe
function of a platform device and is only freed in the remove function.

Additionally, this patch removes a null check after platform_get_resource that
is redundant with the one done by devm_request_and_ioremap.

Signed-off-by: Damien Cassou <damien.cassou@lifl.fr>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>