I don't have the same monitor but I don't have any problem lighting up with
HDMI 2.0 4k 60Hz with Alex's amd-staging-4.7 and the commit from a week ago:

commit 4fa58023f62a95e93f82a25cdceb87b19b69b27f
Author: Christian König <christian.koenig@amd.com>
Date:   Mon Oct 17 15:34:07 2016 +0200

    reservation: revert "wait only with non-zero timeout specified (v3)"

    This reverts commit fb8b7d2b9d80e1e71f379e57355936bd2b024be9.

    Otherwise signaling might never be activated on the fences. This can
    result in infinite waiting with hardware which has unreliable interrupts.

    Signed-off-by: Christian König <christian.koenig@amd.com>
    Reviewed-by: Chunming Zhou <david1.zhou@amd.com>

If you still see this issue please attach xorg log, dmesg and xrandr output, as
Alex suggested.