vfio/display: Fix missing update to set backing fields

vfio/display: Fix missing update to set backing fields

The below referenced commit renames scanout_width/height to
backing_width/height, but also promotes these fields in various portions
of the egl interface.  Meanwhile vfio dmabuf support has never used the
previous scanout fields and is therefore missed in the update.  This
results in a black screen when transitioning from ramfb to dmabuf display
when using Intel vGPU with these features.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1891
Link: https://lists.gnu.org/archive/html/qemu-devel/2023-08/msg02726.html
Fixes: 9ac06df8b684 ("virtio-gpu-udmabuf: correct naming of QemuDmaBuf size properties")
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Tested-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>