drm/amd/display: Fix compile error on older GCC versions

GCC 4.9 reports a 'missing braces around initializer' error. This is a
bug, documented here:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119



Fix it by adding another brace.

Signed-off-by: Leo (Sunpeng) Li <sunpeng.li@amd.com>
Reviewed-by: Mikita Lipski <Mikita.Lipski@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>