vp9: Disable re_encode_overshoot feature for speed >= 6.

vp9: Disable re_encode_overshoot feature for speed >= 6.

For real-time screen content mode: for speed >= 6 disable
the re_encode_overshoot feature. This means for speed >= 6
the Q and rate control is reset on slide changes based on
the scene/slide detection and the current Q (and not on a
first pass encoded frame at current Q).

This reduces encode time on slide changes, but may be less
accurate in deciding when to reset/max-out the Q.

Change-Id: Id0fdcafd55bc43bd8b3afee211e524f37c8ddce6