V4L/DVB: v4l: mem2mem_testdev: fix g_fmt NULL pointer dereference

Calling g_fmt before s_fmt resulted in a NULL pointer dereference as no
default formats were being selected on probe.

Reported-by: Németh Márton <nm127@freemail.hu>
Signed-off-by: Pawel Osciak <p.osciak@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>