Change setDirectRenderSurface to return the result

No need to hide the bool; it can be useful in compositors to check if
direct rendering was properly activated.

Change-Id: I4c12c9719aa6b7b5ec4a91602c5052b6841e1780
Reviewed-by: Jan Arne Petersen <jan.petersen@kdab.com>
Reviewed-by: Andy Nichols <andy.nichols@digia.com>