Correctly handle stopping and restarting the encoder, keeping
track of the stop and drain states.

In addition it adds correct handling of corner cases, allowing
v4l2-compliance to pass.

Unfortunately, the code is getting to be quite complicated, so
we need to work on better codec support in v4l2-mem2mem.c to
simplify drivers.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>