[media] em28xx: move videobuf2 related data from struct em28xx to struct v4l2

The video and VBI-specific VB2 queue and mutexes are used only by
the v4l2 sub-driver. Move them to the V4L2 struct, preventing
wasting memory if this sub-driver is not used.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>