ARM: dts: qcom-msm8974: rename mdss node to display-subsystem

Follow the schema change and rename mdss node to generic name
'display-subsystem'.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230109051402.317577-5-dmitry.baryshkov@linaro.org