arm64: dts: qcom: sdm845: make DP node follow the schema

Drop the #clock-cells (probably a leftover from the times before the DP
PHY split)

Fixes: eaac4e55

 ("arm64: dts: qcom: sdm845: add displayport node")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230110042126.702147-1-dmitry.baryshkov@linaro.org