Comment # 1 on bug 1094751 from Takashi Iwai

One thing you may check quickly is to test kernel-vanilla package.
Just install kernel-vanilla package from the standard Leap 15.0, boot with
vanilla kernel and retest.

It corresponds to the unpatched 4.12.14, while Leap kernel contains lots of
i915 backports that is almost equivalent with 4.16.

If kernel-vanilla is OK, it's likely a regression by the backports.