drm/vkms: Use wait_for_flip_done

It's the recommended version, wait_for_vblanks is a bit a hacky
interim thing that predates all the flip_done tracking. It's
unfortunately still the default ...

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Cc: Haneen Mohammed <hamohammed.sa@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190719152314.7706-2-daniel.vetter@ffwll.ch