Surface: Ensure synchronisation of copyBlt

The Surface::lock() function has an optimisation for copying the
previously drawn contents from the frontbuffer to the (current)
backbuffer, so that the client does not have to redraw the whole surface
contents. This logic was not using the sync fence FD from
dequeueBuffer() and so was not correctly synchronised with pending
operations on the backbuffer.

Change-Id: Ib44574d50f8bdb4a23078cd8da1c5c512c876320