[media] s5p-mfc: Kill all IS_ERR_OR_NULL in clocks management code

After commit "s5p-mfc: Fix clock management in s5p_mfc_release function"
all clocks related functions are called only when MFC device is really
available, so there is no additional check needed for NULL
gate clocks. This patch simplifies the code and kills IS_ERR_OR_NULL
macro usage.

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>