Displays are always noisy. In the DS turning it off (using the Dim button) makes a noticeable difference. I expect that it will on the P20 as well. I always run with the display “dimmed” on both.
Most displays require voltages other than just 5V so there is usually some switching regulator to generate those voltages which will radiate and also cause noise on the input power. The speed of the dots turning on and off is high enough that there is radiation from the display driver. The loops in a processor which is driving a display can also cause noise in the power of the processor and radiation…