[media] v4l: mem2mem_testdev: Add horizontal and vertical flip

Add horizontal and vertical flip to the mem2mem_testdev driver.
Flip modes can be enabled either separately or simultaneously.

Signed-off-by: Tomasz Moń <desowin@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>