media: v4l2-ioctl: Zero v4l2_plane_pix_format reserved fields

Make the core set the reserved fields to zero in
vv4l2_pix_format_mplane.4l2_plane_pix_format,
for _MPLANE queue types.

Moving this to the core avoids having to do so in each
and every driver.

Suggested-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>