dt-bindings: msm: disp: remove bus from dpu bindings

Driver and dts has been already adjusted and bus moved out of dpu, let's
update also dt-bindings.

Fixes warnings as:
arch/arm64/boot/dts/qcom/sdm845-oneplus-fajita.dt.yaml: mdss
@ae00000: clock-names: ['iface', 'core'] is too short
        From schema: Documentation/devicetree/bindings/display/msm/dpu-sdm845.yaml

Ref: https://lore.kernel.org/all/20210803101657.1072358-1-dmitry.baryshkov@linaro.org/



Signed-off-by: David Heidelberg <david@ixit.cz>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211220184220.86328-1-david@ixit.cz