vp9: Adjust intra check for short_circuit_flat_blocks

For non-rd pickmode: include H and V intra mode check for
spatially flat blocks when the sf->short_circuit_flat_blocks
speed feature is set.
Small improvement on screen content tests.

Change-Id: I3391d02cce6a46160be6ccc8a1e33fd8547eb467
1 file changed