v4l2allocator: Workaround driver that don't support REQBUFS(0)

v4l2allocator: Workaround driver that don't support REQBUFS(0)

There is still around 18 drivers not yet ported to videobuf2. These driver
don't support freeing buffetrs through REQBUFS(0) hence for these the
memory type probing fails. In order to gain back our previous behaviour in
presence of these, we implement a workaround that assuming MMAP is
supported. Note that an allocator is only created for device with
STREAMING support in the device capabilities. In such case one of MMAP,
USERPTR and DMABUF is required. Though DMABUF came afterward, so is
not an option and in practice none of these drivers will only do USERPTR.

https://bugzilla.gnome.org/show_bug.cgi?id=735660

Also-by: Hans de Goede <hdegoede@redhat.com>