[media] exynos-gsc: Make PM callbacks available conditionally

There are no need to set up the PM callbacks (runtime and system) unless
they are being used. It also causes compiler warnings about unused
functions.

Silence the warnings by making them available for CONFIG_PM (runtime
callbacks) and CONFIG_PM_SLEEP (system sleep callbacks).

[mszyprow: squashed two patches into one to avoid potential build
break, changed patch subject and updated commit message]

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
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>