dt-bindings: msm: dsi: Fix power-domains constraint

The existing msm8916.dtsi does not depend on nor require power-domains.
Drop from the list of required.

Fixes: 4dbe55c9 ("dt-bindings: msm: dsi: add yaml schemas for DSI bindings")
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>