hevc: compact ff_hevc_luma_mv_mvp_mode

LX can be either 0 or 1.
hevc: unlispify ff_hevc_luma_mv_mvp_mode
hevc: uniform variables in ff_hevc_luma_mv_mvp_mode
hevc: use macros to simplify
hevc: Use macros part 2
hevc: Wrap compareMVrefidx in a macro
hevc: Simplify check_prediction_block_available
hevc: Clean up isDiffMER
hevc: Refactor derive_temporal_colocated_mvs
hevc: Use macro to simplify derive_temporal_colocated_mvs
hevc: Unlispify temporal_luma_motion_vector
hevc: Use macro to simplify temporal_luma_motion_vector
hevc: Refactor derive_spatial_merge_candidates
hevc: Clean up ff_hevc_luma_mv_merge_mode
hevc: Factor out motion vector scaling
hevc: Clean up dist_scale