media: imx-jpeg: Implement drain using v4l2-mem2mem helpers

v4l2 m2m has supplied some helper function to handle drain,
so the driver can use the helper function directly.

Fixes: d8ebe298 ("media: imx-jpeg: Set V4L2_BUF_FLAG_LAST at eos")
Signed-off-by: Ming Qian <ming.qian@nxp.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>