media: videodev2.h: Move HM12 format to YUV semi-planar section

V4L2_PIX_FMT_HM12 is a YUV semi-planar macro-block format. Move it from
the packed YUV formats section where it was misplaced to the YUV
semi-planar formats section.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>