blast: remove unnecessary compute bound computations

mVisibleRegionsDirty (caused by sequence++) should only be set
when the bounds of layers are affected by a change in state.

This change improves performance by saving us from unnecessary
calls.

Test: SurfaceFlinger_test
Bug: N/A
Change-Id: I822fdfb82cb3cd0bbc3d4bb6685f06750e152dca