Decouple macroblockd_plane buffer usage

Make the macroblockd_plane contain dynamic buffer pointers instead
static pointers to the memory space allocated therein. The decoder
uses the buffer allocated in pbi, while encoder will use a dual
buffer approach for rate-distortion optimization search.

Change-Id: Ie6f24be2dcda35df7c15b4014e5ccf236fb3f76c