libgui: Fix CPU rendering on Surface

When the surface damage code went in, it incorrectly assumed that if
an application was doing CPU rendering, it would be using lock and
unlockAndPost instead of dequeue and queue, so it repurposed the dirty
region too aggressively. This change keeps it from clobbering the
dirty region if a CPU producer is attached.

Bug: 20431815
Change-Id: Id4dfd71378311ea822f0289f6de2d20a7bd84014