dt-bindings: msm: dsi-controller-main: Fix power-domain constraint

power-domain is required for the sc7180 dispcc GDSC but not every qcom SoC
has a similar dependency for example the aqp8064.

Most Qcom SoC's using mdss-dsi-ctrl seem to have the ability to
power-collapse the MDP without collapsing DSI.

For example the qcom vendor kernel commit for apq8084, msm8226, msm8916,
msm8974.

https://review.carbonrom.org/plugins/gitiles/CarbonROM/android_kernel_oneplus_msm8994/+/7b5c011a770daa2811778937ed646237a28a8694



"ARM: dts: msm: add mdss gdsc supply to dsi controller device

 It is possible for the DSI controller to be active when MDP is
 power collapsed. DSI controller needs to have it's own vote for
 mdss gdsc to ensure that gdsc remains on in such cases."

This however doesn't appear to be the case for the apq8064 so we shouldn't
be marking power-domain as required in yaml checks.

Fixes: 4dbe55c9 ("dt-bindings: msm: dsi: add yaml schemas for DSI bindings")
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>