[media] exynos4-is: Simplify sclk_cam clocks handling

Use clk_prepare_enable()/clk_disable_unprepare() instead of
separately prearing/unparing the clk_cam clocks. This simplifies
the code that is now mostly not going to be used, function
__fimc_md_set_camclk() is only left for S5PV210 platform which
is not yet converted to Device Tree.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>