There are 4 delta_lflevels (FRAME_LF_COUNT=4), corresponding to 2 for the luma
plane and 1 for each of the U and V planes. In monochrome mode we do not need
to use/encode/decode the last two values, and so in this case the for loops all
terminate at FRAME_LF_COUNT - 2 rather than FRAME_LF_COUNT.

BUG=aomedia:1410

Change-Id: Ieb60cce9e39287a385e7f74d9009aac1f0f23e54