workaround [3201922] display not on: log full of gralloc errors

while we're waiting for the real fix in the gralloc/gpu driver,
this workaround should resolve the issue.

we make sure to call compositionComplete after everytime we do
composition with the GPU (even for the screenshot case), which is
where the buffer locks are released.

Change-Id: I3cb5ad67d48c81a23100172bab77e86a70e29152