Rework the format selection on the source pad. Make the format
on the source pad selectable amongst a list of formats. This
list can be different for each sink pad format.

Signed-off-by: Todor Tomov <todor.tomov@linaro.org>
Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>