Fix use of scaling in joint motion search

To enable us to the scale-invariant motion estimation
code during mode selection, each of the reference
buffers is scaled to match the size of the frame
being encoded.

This fix ensures that a unit scaling factor is used in
this case rather than the one calculated assuming that
the reference frame is not scaled.

Change-Id: Id9a5c85dad402f3a7cc7ea9f30f204edad080ebf