[media] v4l: mem2mem: port m2m_testdev to vb2

This patch ports mem2mem test device to videobuf2 framework.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>