Surface: Use async lock/unlock in copyBlt

The Surface::lock() function now uses the asynchronous versions of
gralloc lock/unlock when copying the previously drawn content to the
backbuffer. This allows for optimisations in the gralloc module
implementation and so can improve performance and avoid CPU waits.

Change-Id: I57193f327db2ff0422e1b58b3484f613201d994c