drm/vmwgfx: Fix a screen object framebuffer dirty corner case

If there are no cliprects for a particular crtc, an invalid command would
have been generated. If that's the case, instead ditch the generated
command sequence.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>