Improve loopfilter function

This patch continued the work done in "Rewrite loop_filter_info_n
struct"(commit:00dbd369) to further
improve loopfilter function.

1. Instead of storing pointers to thresholds, store loopfilter
levels within 64x64 SB;
2. Since loopfilter levels are already calculated in setup_mask,
we don't need call build_lfi to look up them again. Just save
loopfilter levels in setup_mask.
3. Reorganized and simplified filter_block_plane().

Tests showed a ~0.8% decoder speedup.

Change-Id: I723c7779738bbc2afcb9afa2c6f78580ee6c3af7