we need to guarantee that h/w comp set() is called when a buffer has been retired

removed a test that could violate this guarantee. note that we
have no proof tha this ever happened, but consequences could be
hard lock-ups.

the code here was intended to track the region to update for displays
that can do partial update. the logic discarded the update entirely
if that region was empty. instead we just redraw the whole thing
(note that we should never be there with an empty region and retired
buffers though).

Bug: 5466259

Change-Id: I91ccab3b1a599e729e438eb833939e2236da6854