media: allegro: remove custom drain state handling

The v4l2-m2m has various helpers for correctly handle the draining. Drop
the driver specific state machine and use the m2m helper functions.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>