[media] s5p-mfc: Ensure that clock is disabled before turning power off

Move clock disabling before turning power off.  This will enable later
to add calls to clk_prepare/unprepare in the s5p_mfc_power_off() function
to avoid keeping clocks prepared all the time when driver is bound.

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>