[media] s5p-fimc: Fix compiler warning in fimc-lite.c

This patch is an update to changed media_entity_pipeline_start()
signature in commit af88be38,
"media: Add link_validate() op to check links to the sink pad"

It fixes the following warning:

drivers/media/video/s5p-fimc/fimc-lite.c: In function ‘fimc_lite_streamon’:
drivers/media/video/s5p-fimc/fimc-lite.c:765:29: warning: ignoring return value
of ‘media_entity_pipeline_start’, declared with attribute warn_unused_result [-Wunused-result]

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>