[media] mem2mem_testdev: Use devm_kzalloc() in probe

devm_kzalloc() makes error handling and cleanup simpler.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>