libvpx

Enable early termination in uv rd loop

This commit enables early termination in the rate-distortion
optimization search loop for chroma components. When the cumulative
rd cost is above the current best value, skip the rest per-block
transform/quantization/coeff_cost and continue to the next
prediction mode.

For bus_cif at 2000 kbps, the average run-time goes down from
168546ms -> 164678ms, (2% speed-up) at speed 0
 36197ms ->  34465ms, (4% speed-up) at speed 1

Change-Id: I9d3043864126e62bd0166250d66b3170d520b3c0