[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().