vp9: Force hybrid_intra on scene change

vp9: Force hybrid_intra on scene change

For real-time screen content mode: when scene/slide change
is detected and re-encode is decided, force hybrid_intra
mode search if slide change is big and alot of Intra modes
were used. hybrid_intra mode will use rd-based intra mode
search for small blocks.

Overall better PSNR on clip with slide changes, with similar
encoded frame size. Encode time lightly higher on average with
this change.

Change-Id: I503835253b777b9f98d74e75a52a8000b76c310c