Make fi mode index entropy coded instead of using raw bits. Mode
cost estimation in key-frame RDO is updated as well. Modification
to inter frame RDO is not included in this patch.
Also key-frame y mode cdf table is re-trained since fi modes are
attached to DC_PRED

Key frame BDRate:
-0.399% lowres, -0.339% midres

Change-Id: I9ccf478b0a2e48fb1870fe8451e45e2c858a5f63