In multi-resolution encoding, frame_type decision for each frame
is made by the lowest-resolution encoder. For all other higher-
resolution encoders, kf_mode is always set to VPX_KF_DISABLED,
and they are forced to use the same frame_type picked by the
lowest-resolution encoder.

Change-Id: Ic4d52ec65bbc012ca9c2d236210e28a295591eaf