drm/amd/powerplay: Use designated initializers

The randstruct plugin requires designated initializers for structures
that are entirely function pointers.

Cc: Christian König <christian.koenig@amd.com>
Cc: Eric Huang <JinHuiEric.Huang@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Kees Cook <keescook@chromium.org>