SurfaceFlinger: recompute visible regions less

This change removes some visible region recomputation that was needed to handle
the SCALING_MODE_FREEZE cropping.  We've changed things to use a window crop
from the WindowManager instead, so this is no longer needed.

Bug: 6299171
Change-Id: I32fbc2b689c985837126d8ba3d9a91e79613ffbf