drm/vmwgfx: Start diffing new mob cursors against old ones

Avoid making the SVGA device do extra work if the new cursor image
matches the old one.

Signed-off-by: Michael Banack <banackm@vmware.com>
Signed-off-by: Zack Rusin <zackr@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221022040236.616490-8-zack@kde.org