arm64: dts: qcom: use DT label for DSI outputs

arm64: dts: qcom: use DT label for DSI outputs

Instead of keeping a copy of the DT tree going down to the DSI output
endpoint use the label to reference it directly, making DTs less
error-prone.

Reviewed-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20250815-msm-dsi-outs-v2-1-3662704e833f@oss.qualcomm.com
Signed-off-by: Bjorn Andersson <andersson@kernel.org>