[media] s5p-mfc: Staticize some symbols in s5p_mfc_cmd_v5.c

These symbols are used only in this file and can be made static.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>