Compound inter encoder bug fix.

In the longer term the encoder should allow compound as long
as one of the buffers has opposite sign bias and as per the decoder
this buffer is then set as the fixed reference. However at the moment
the encoder and RD loop only supports the case where the ALTREF_FRAME
buffer (or third of the 3 allowed in any given frame) is the odd one out.

This patch fixes a bug that would allow compound inter and set
fixed ref to ALTREF_FRAME when it is not the odd one out.

Change-Id: Ic83a69486e088a147ba83a4aedc2a0042f6b3721