[media] s5p-fimc: Use video entity for marking media pipeline as streaming

It doesn't matter whether we start from the sensor of from
the video node entity. Remove use of pipeline->subdevs array
where possible, so we can partly drop dependency on struct
fimc_pipeline in the fimc-lite module, which is also used
by the exynos5-is driver.
Also make sure we revert any media entity pipeline operations
when vb2_streamon() function fails.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>