[media] v4l: mem2mem_testdev: Fix race conditions in driver

The mem2mem_testdev allows multiple instances to be opened in parallel.
Source and destination queue data are being shared between all
instances, which can lead to kernel oops due to race conditions (most
likely to happen inside device_run()).

Attached patch fixes mentioned problem by storing queue data per device
context.

Signed-off-by: Tomasz Moń <desowin@gmail.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>