Revert "x11-drivers/xf86-video-amdgpu: port to EAPI 7"

Revert "x11-drivers/xf86-video-amdgpu: port to EAPI 7"

In the port to EAPI=7, we stopped inheriting xorg-2 since that eclass is
not EAPI 7 ready. Unfortunately it was providing some critical
functionality:

[[ ${PN} == xf86-video-* || ${PN} == xf86-input-* ]] && DRIVER="yes"
...
xorg-2_flags_setup() {
...
[[ ${PN} = xorg-server || -n ${DRIVER} ]] && append-ldflags -Wl,-z,lazy

In order to revert this commit, we must revert some commits that fix
problems introduced by it as well. In total:

This reverts commit 568eca2a556b1f5ba09d5486374cec7346ab1fce.

Revert "x11-drivers/xf86-video-amdgpu: add --disable-selective-werror"

This reverts commit 66c24d866e939044601f58d1521e00224ca8bf4b.

Revert "x11-drivers/xf86-video-amdgpu: fix xorg-server:= dependency"

This reverts commit e900d1be730882c962354203277ebb1ba499c0d3.

Closes: https://bugs.gentoo.org/661502