Don't call hwc set() if there is nothing new to do

there was situations where SF's main loop would run (as if there was
an invalidate), but the dirty region was empty (so no new buffers
were retired). In this case we return early and don't swap, which
would cause drawing artifacts.

Bug: 5476838
Change-Id: Id3b7bf4b7aabec7919c50d9278eb2165973a4c3d