[media] s5p-mfc: Fix clock management in s5p_mfc_release() function

Clock control indirectly requires access to MFC device, so call it only
if we are sure that the device exists in s5p_mfc_release function.
s5p_mfc_remove() calls s5p_mfc_final_pm(), which releases all PM related
resources, including clocks, so any call to clocks related functions
is not valid after s5p_mfc_final_pm().

Fixes: d695c12c ("[media] media: s5p-mfc fix invalid memory access from
s5p_mfc_release()")

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>