drm/vgem: add compat_ioctl support

DRM drivers should supply a compat version if they're going to provide
an ioctl implementation at all. This can confuse 32-bit user space on a
64-bit system.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170715031212.108695-1-briannorris@chromium.org