LE supports a single GPU being used, and if you have multiple devices the one used is determined through udev, see:
LibreELEC.tv/97-xorg.rules at master · LibreELEC/LibreELEC.tv · GitHub
^ the i915 device will be detected first, the amdgpu or radeon drivers are later. The simplest option to prevent i915 from being used is to blacklist the kernel module so it's not loaded at boot, e.g.
echo "blacklist i915" > /storage/.config/modprobe.d/i915.conf
This will prevent udev seeing the node from the driver and it should find the AMD card instead.
NB: the above assumes i915 is the module name. It's about 10-years since I booted LE on anything with an Intel GPU so check it's right first.