surfaceflinger: remove DisplayDevice::swapRegion

With SWAP_RECTANGLE and PARTIAL_UPDATES removed, it becomes clear
that swapRegion is not needed.  It also becomes clear that
DisplayDevice::flip and SurfaceFlinger::doComposeSurfaces do not
need the dirty region.

Test: SurfaceFlinger_test
Change-Id: Id9cd2b43812d3c7dcfa36f605d0f8a647264f228