arm64: dts: qcom: sm8550: fix DSI controller compatible

Add missing sm8550 soc specific compatible before fallback to
match the updated bindings.

Fixes: d7da51db ("arm64: dts: qcom: sm8550: add display hardware devices")
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230207-topic-sm8550-upstream-sm8550-dt-fix-v1-1-698d132ab285@linaro.org