[media] s5p-cec: fix system and runtime PM integration

Use generic helpers instead of open-coding usage of runtime PM for system
sleep PM, which was potentially broken for some corner cases.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>