[media] s5p-fimc: Make 'fimc_pipeline_s_stream' function static

Fixes the following sparse warning:
drivers/media/platform/s5p-fimc/fimc-mdevice.c:216:5: warning:
symbol 'fimc_pipeline_s_stream' was not declared. Should it be static?

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