arm64: dts: qcom: msm8916/39: Clean up MDSS labels

Right now MDSS related definitions cannot be properly grouped together
in board DTs because the labels do not use consistent prefixes. The DSI
PHY label is particularly weird because the DSI number is at the end
(&dsi_phy0) while DSI itself is called &dsi0.

Follow the example of more recent SoCs and give all the MDSS related
nodes a consistent label that allows proper grouping.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230525-msm8916-labels-v1-4-bec0f5fb46fb@gerhold.net