staging: vboxvideo: Add vboxvideo to drivers/staging

This commit adds the vboxvideo drm/kms driver for the virtual graphics
card used in Virtual Box virtual machines to drivers/staging.

Why drivers/staging? This driver is already being patched into the kernel
by several distros, thus it is good to get this driver upstream soon, so
that work on the driver can be easily shared.

At the same time we want to take our time to get this driver properly
cleaned up (mainly converted to the new atomic modesetting APIs) before
submitting it as a normal driver under drivers/gpu/drm, putting this
driver in staging for now allows both.

Note this driver has already been significantly cleaned up, when I started
working on this the files under /usr/src/vboxguest/vboxvideo as installed
by Virtual Box 5.1.18 Guest Additions had a total linecount of 52681
lines. The version in this commit has 4874 lines.

Cc: vbox-dev@virtualbox.org
Cc: Michael Thayer <michael.thayer@oracle.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Michael Thayer <michael.thayer@oracle.com>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>