vp9: limit lpf workers to min(threads,tiles,sb_rows)
vp9: limit lpf workers to min(threads,tiles,sb_rows)

this implementation does not scale well beyond that. this restores the
performance in v1.7.0.

BUG=webm:1574

Change-Id: I8f3464cfe871988fa06ebefe9954811fd002584e

ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx