Make back buffers non-lockable.

Bug 21716622

Change-Id: I041fd37d461114a52a3b3dfa57e4aee6c01e5ee5
Reviewed-on: https://swiftshader-review.googlesource.com/3445
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>