drm/virtgpu: fix double unregistration

drm_put_dev also calls drm_dev_unregister, so dev will be unregistered
twice.
Replace it with drm_dev_put to fix it.

Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191109075417.29808-1-hslester96@gmail.com