[media] exynos4-is: Improve clock management

There is no need to keep all clocks prepared all the time. Call to
clk_prepare/unprepare can be done on demand from runtime pm callbacks
(it is allowed to call sleeping functions from that context).

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