[media] exynos-gsc: Simplify clock management

Instead of having separate functions that fetches, prepares and
unprepares the clock, let's encapsulate this code into ->probe().

This makes error handling easier and decreases the lines of code.

[mszyprow: rebased onto v4.9-rc4]

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>