[AMDGPU] Fix std::array initializers to avoid warnings with older tool chains. NFC

A std::array is implemented as a template with an array
inside a struct. Older versions of clang, like 3.6,
require an extra set of curly braces around std::array
initializations to avoid warnings.

The C++ language was changed regarding this by CWG 1270.
So more modern tool chains does not complaing even if
leaving out one level of braces.

llvm-svn: 361171