staging/vboxvideo: Use same BO mmap offset as other drivers

The vboxvideo driver mmaps BOs at 0x10000000 or higher. Changing the
offset to 0x100000000 aligns the driver with all other DRM drivers.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>