media: v4l2-mem2mem: reorder checks in v4l2_m2m_poll()

When reaching the end of stream, V4L2 m2m clients may expect the
V4L2_EOS_EVENT. Although the V4L2_EOS_EVENT is deprecated behavior,
drivers must signal that event before dequeuing the buffer that has the
V4L2_BUF_FLAG_LAST flag set.

If a driver queues the V4L2_EOS_EVENT event and returns the buffer after
the check for events but before the check for buffers, vb2_m2m_poll()
will signal that the buffer with V4L2_BUF_FLAG_LAST can be read but not
that the V4L2_EOS_EVENT is available.

Split the check for buffers into a separate function and check for
available buffers before checking for events. This ensures that if
vb2_m2m_poll() signals POLLIN | POLLRDNORM for the V4L2_BUF_FLAG_LAST
buffer, it signals POLLPRI for the V4L2_EOS_EVENT, too.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
[hverkuil-cisco@xs4all.nl: fix checkpatch alignment warning]
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>