[media] videobuf: Allow reqbufs(0) to free current buffers

All the infrastructure for this is already there, and despite our desires for
the old videobuf code to go away, it is currently still in use in 18 drivers.

Allowing reqbufs(0) makes these drivers behave consistent with modern drivers,
making live easier for userspace, see e.g. :
https://bugzilla.gnome.org/show_bug.cgi?id=735660

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>