drm/vmwgfx: Plane atomic state

Add plane state handling functions.

We have to keep track of a few plane states so we cannot use the
DRM helper for this.

Created vmw_plane_state along with functions to reset, duplicate,
and destroty it.

v2
* Removed cursor clean up special case

Signed-off-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Acked-by: Daniel Vetter <daniel@ffwll.ch>