[media] v4l2: vb2-dma-sg: fix memory leak

This patch fixes two minor memory leaks in videobuf2-dma-sg module. They
might happen only in case some other operations (like memory allocation)
failed.

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