staging: vboxvideo: Temporarily remove page_flip support

drm_mode_page_flip_ioctl() cannot deal with the in between phase of
the transitioning to atomic modeset support. Once we start using
drm_helper_crtc_mode_set(), we start setting plane->state on the primary
plane. But we are not fully atomic yet so then set both plane-state->fb
and plane->fb.

If both plane-state->fb and plane->fb are set drm_mode_page_flip_ioctl()
gets confused and stops calling drm_framebuffer_get() on the new fb while
still calling drm_framebuffer_put() on the old fb.

The current page_flip implementation expects drm_mode_page_flip_ioctl()
to take care of both and once we switch to drm_atomic_helper_page_flip()
that will expect neither to be done, taking care of both itself.

So for the transition we need to remove page_flip support and then after
the transition is complete and we set DRIVER_ATOMIC in our driver_features,
we can start using drm_atomic_helper_page_flip().

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>