staging: vboxvideo: Keep old mode when disable crtc

On DPMS off we get a call to vbx_crtc_atomic_disable, followed by
a call to vbox_primary_atomic_update, at which point crtc_state->enable
is 0 and crtc_state->mode has been zero-ed. On a 0 width/height
vbox_do_modeset() falls back to 640x480, so this causes the guest Window
(its "monitor") to resize to 640x480 (and become black).

This commit makes us not look at crtc_state->mode when crtc_state->enable
is not set, so that we keep the old mode and just make the window go black.

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