libvpx

Initialize internal buffer pointers in rtc mode

In rtc coding mode, the encoder is running non-RD mode decision. It
does not need dual buffer swap as was the case in the RD mode. This
commit initializes the internal buffer pointers outside the block
coding loop for rtc mode.

Change-Id: Ie076705c60d6b7919217e3f1dfd49e7db5064ac2