media: v4l2-mem2mem: add new_frame detection

Drivers that support VB2_V4L2_FL_SUPPORTS_M2M_HOLD_CAPTURE_BUF
typically want to know if a new frame is started (i.e. the first
slice is about to be processed). Add a new_frame bool to v4l2_m2m_ctx
and set it accordingly.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>