Downstream bug: https://bugs.gentoo.org/698114
If configured with --disable-glamor, xf86-video-ati fails to build with the following error:
radeon_present.c: In function ‘radeon_present_check_flip’:
radeon_present.c:281:21: error: dereferencing pointer to incomplete type ‘struct radeon_pixmap’
if (priv && priv->fb_failed)
This is probably caused by  which was backported from xf86-video-amdgpu and requires glamor to be enabled.