[media] exynos4-is: Ensure fimc-is clocks are not enabled until properly configured

Use clk_prepare_enable/clk_unprepare_disable instead of preparing the
clocks during the driver initalization and then using just clk_disable/
clk_enable. The clock framework doesn't guarantee a clock will not get
enabled during e.g. clk_set_parent if clk_prepare has been called on it.
So we ensure clk_prepare() is called only when it is safe to enable
the clocks, i.e. the parent clocks and the clocks' frequencies are set.
It must be ensured the FIMC-IS clocks have proper frequencies before they
are enabled, otherwise the whole system will hang.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyunmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>