As discovered in path 10 of Change Ia12acd2f, reset 2nd order coeffs
without reset of above and left coding context may have introduced
problem that causes encoder/decoder mismatching. This commit added
update to coding context when the 2nd coefficients are cleared.

In addition, this commit also introduced early breakout in the checks
to speed up when coefficients are too significant to be cleared.

Change-Id: I85322a432b11e8af85001525d1e9dc218f9a0bd6