Use the previously introduced v4l2-mem2mem core APIs to handle the drainig,
stopped and next-buf-is-last states.

With these changes, the v4l2-compliance still passes with the following
commands :
>>>><><><><><><><><><><><><><><><><>< 15.53 fps
 15.53 fps
><><><><><><><><><><><><>< 13.99 fps
 13.99 fps
><><><><><><><><><><><>< 13.52 fps
 13.52 fps
><><><><><><><><><><><><>< 13.41 fps
 13.41 fps
><><><><><><><><><><><><>< 13.21 fps
 13.21 fps
><><><><><><><><><><><>< 13.09 fps
 13.09 fps
><><><><><><><
STOP ENCODER
<<<
EOS EVENT

v4l2-compliance SHA: 7ead0e1856b89f2e19369af452bb03fd0cd16793, 64 bits
[...]
Total for vicodec device /dev/video0: 50, Succeeded: 50, Failed: 0, Warnings: 0

The full output is available at [1]

v4l2-compliance SHA: 7ead0e1856b89f2e19369af452bb03fd0cd16793, 64 bits
[...]
Total for vicodec device /dev/video1: 50, Succeeded: 50, Failed: 0, Warnings: 0

The full output is available at [2]

No functional changes should be noticed.

[1] https://termbin.com/25nn
[2] https://termbin.com/dza4Suggested-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Suggested-by: Maxime Jourdan <mjourdan@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>