drm/vmwgfx: Properly flush cursor updates and page-flips

With the introduction of the new command buffer mechanism,
proper care wasn't taken to flush cursor image updates and
event-less screen-target page-flips.

Fix this by introducing explicit flush points.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>