Mesa lacks video_cards_amdgpu USE flag, but even setting video_cards_radeonsi it's impossible remove all radeon dependecies: >=x11-libs/libdrm-2.4.96[video_cards_radeon] required by (media-libs/mesa-18.3.1:0/0::gentoo, installed) Reproducible: Always
Created attachment 560064 [details] emerge --info
Yes, I also encountered this when setting up. Flag "video_cards_amdgpu" really could use it!
Sorry, it's not possible. See https://bugs.gentoo.org/697552#c1