Speed up recursive transform block partition search. When a txfm
block is selected as all zero coefficients, skip the search over
further split partition.

Tested with txk-sel on, this makes the speed 0 / 1 both 10 - 15%
faster at medium - high target bit-rate range. The coding
performance change is neutral - 0.011% better for lowres set.

Change-Id: I1247f3d5a33d15bf4bc5f0bcbac2bf1f3e1aca2e