don't clear the framebuffer when the framebuffer is not used

when areas of the FB are undefined (transparent windows on top of
nothing), we clear those areas before composition.
however, it makes no sense to do this when the FB is not in use
(case where hwc handles all layers)

Bug: 5360529
Change-Id: I761a7d3664228ebbee0944aca8b0cc278fc94c9c