Using only one scale function in scale_factors struct.

Functions scale_mv_q4 and scale_mv_q3_to_q4 were almost identical except
q3->q4 conversion in scale_mv_q3_to_q4. Now q3->q4 conversion happens
directly in vp9_build_inter_predictor.

Also adding useful constants: SUBPEL_BITS and SUBPEL_MASK.

Change-Id: Ia0a6ad2ac07c45fdf95a5139ece6286c035e9639