drm/vmwgfx: Cursor update fixes

Use drm_plane_helper_check_update also for the cursor plane.
Some applications, like gdm on gnome shell still uses cursor front-buffer
like rendering without notifying the kernel. We do need some kind of
noficiation, but work around this for now by updating the cursor image on
every cursor move.

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