drm/vkms: vkms_driver can be static

Fixes: 58d8108f080c ("drm/vkms: Introduce basic VKMS driver")
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Acked-by: Haneen Mohammed <hamohammed.sa@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180515113052.GA111532@lkp-ib04