virtio_gpu_mode_dumb_destroy() is the same as drm_gem_dumb_destroy()
which is the drm_driver.dumb_destroy default, so no need to set it.

Cc: David Airlie <airlied@linux.ie>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1502034068-51384-19-git-send-email-noralf@tronnes.org