drm_hwcomposer: Use the global default cpp_std.
Avoid overriding the default cpp_std
Bug: http://b/344590580 Signed-off-by: Elliott Hughes enh@google.com [jstultz: Tweaked commit message] Signed-off-by: John Stultz jstultz@google.com Change-Id: I6bde2ab4d208c990103e6d303ec96a4969fa06ef