[media] s5p-fimc: Update graph traversal for entities with multiple source pads

We cannot assume that the passed entity the fimc_pipeline_prepare()
function is supposed to start the media graph traversal from will
always have its sink pad at pad index 0. Find the starting media
entity's sink pad by iterating over its all pads and checking the
pad flags. This ensures proper handling of FIMC, FIMC-LITE and
FIMC-IS-ISP subdevs that have more than one sink and one source pad.

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>