Current mesa git requires >=x11-libs/libdrm-2.4.88 if it is built with the "video_cards_amdgpu" useflag. Unfortunately this version of libdrm is not yet in the Gentoo tree, hence compilation is impossible. Both the mesa ebuilds needs adjusting as well as a new libdrm package. See: https://cgit.freedesktop.org/mesa/mesa/tree/configure.ac#n69
Fixed in commit 955f708700a623adbae8aad8913261f90b015e52 and commit 7d0b1cfda31678c7d85f417c392bf486117c00e6