arm64: dts: qcom: sc8280xp: fix external display power domain

Fix the external display controller nodes which erroneously described
the controllers as belonging to CX rather than MMCX.

Fixes: 19d3bb90 ("arm64: dts: qcom: sc8280xp: Add USB-C-related DP blocks")
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230316141252.2436-1-johan+linaro@kernel.org