If agp module is not loaded before drm/i915 driver modules, serious kernel issues will happen, most notably: 1) display turned OFF 2) frantic kernel crash with no keyboard/network/sysrq availability when starting Xorg (buffers are neither flushed to disk) I think a notice should be added to the help text shown in menuconfig regarding the AGP drivers See also J.Barnes' comment (URL)
Downstream issue at Arch Linux: http://bugs.archlinux.org/task/18690