[media] mem2mem_testdev: pick default format with try_fmt

This resolves an issue raised by v4l2-compliance: if the given format does
not exist, then pick a default format.

While there is an exception regarding this for TV capture drivers, this
m2m driver should do the right thing.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>