VP9: Pass NULL scale_factors ptr when not scaling

to vp9_setup_pre_planes(), preventing the function
unscaled_value() from being called.  unscaled_value()
returns the same value that was passed in.  See
scaled_buffer_offset() in vp9_reconinter.h.

Change-Id: I2a6fbaf07972c2f212834929d29a2cbe72e399c3