drm/amdgpu: add check to avoid array bound issue

Sub_block_index can be passed from user level, so
add one check before accessing the array first to
prevent array index out of bound problem.

Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>