Good question. When it comes to embedded Graphics you need to contact AMD Support via Email. They will hook you up with a Graphics embedded specialist to answer your inquiry. Send email here : Email Form
You are correct that a RX580 requires a Minimum TOTAL SYSTEM WATTAGE PSU of 550 Watts (PSU REQUIREMENTS - RealHardTechX ) if you are using a separate RX580 card on a PCIe x16 slot on your motherboard. But since the circuitry of the RX580 is built into your Motherboard, you need to ask AMD Support directly about your problem.
Went to Dell Support and found the Drivers for your Computer: There are two important Drivers that may help from 2017. I don't know if you have the latest one installed or not. Support for Inspiron 27 7775 | Drivers & downloads | Dell US ..
The BIOS Update is considered to be "OPTIONAL". It supposed to help stability of the computer. Dell Inspiron 5475/7775 AIO System BIOS. Driver Details | Dell US
BIOS 08 Dec 2017
Version: 2.12.0 ,2.12.0 Older versions
Release Date: 08 Dec 2017
File Name: Inspiron_5475_AIO_Inspiron_7775_AIO_x64_2.12.0.exe
File size: 24.32 MB
Description: This package provides the Dell System BIOS update and is supported on Dell Inspiron 5475/7775 AIO for Windows Operating Systems.
View full driver details Video 26 Jul 2017
Version: 17.100.2901 ,A02 Older versions
Release Date: 04 Jul 2017
File Name: AMD-RX560-RX580-R7-Graphics-Controller-GPU_63HD1_WIN_17.100.2901_A02.EXE
File size: 608.02 MB
Description: This package provides the driver for the AMD Radeon TM RX560 RX580 R7 Graphics Controller and is supported Inspiron 5475/7775 Series running the following Windows operating systems: Windows10.