drm/vmwgfx/sou: Use drm_mode_config_reset

Same problem as in stdu, same fix.

Fixes: 51f644b4

 ("drm/atomic-helper: reset vblank on crtc reset")
Acked-by: Charmaine Lee <charmainel@vmware.com>
Reviewed-by: Zack Rusin <zackr@vmware.com>
Signed-off-by: Roland Scheidegger <sroland@vmware.com>