graphics: fix a potential use after free

We cannot lookup _and_ update buffer cache entry in lookupBuffer.
The old buffer is still in use by hwcomposer2.  Add updateBuffer to
do the update after the new buffer has replaced the old buffer in
hwcomposer2.

While at it, s/BufferClone/BufferCacheEntry/g.

Bug: 36064845
Test: manual
Change-Id: I59b61c0198ad528c40020fdebbe27a6cc359226f