When flushing the capture buffers, the driver may send a V4L2_EVENT_EOS
to notify that draining is completed. Currently, v4l2_m2m does not
subscribe to this event, which can cause some devices (i.e. imx8qm) to
hang at the end of encoding/decoding. Support for handling the event is
added in this commit.

Some devices may not signal V4L2_EVENT_EOS. This is logged as a warning
message during initialization and not treated as a fatal error.
Signed-off-by: Ming Qian <ming.qian@nxp.com>
Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>