[media] mem2mem: replace BUG_ON with WARN_ON

See following thread for rationale:
	http://www.spinics.net/lists/linux-media/msg52462.html


Tested by compilation only.

Signed-off-by: Nicolas Thery <nicolas.thery@st.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>