I don't know whether or not the "conflicting fb" message is causing your boot problems, but we can try to work around it. Add this line:
GRUB_GFXPAYLOAD_LINUX=text
to /etc/default/grub, re-run 'sudo update-grub2', and see if you can boot the bisect kernels after that. This should prevent the vesafb driver from trying to use the framebuffer.
I don't know whether or not the "conflicting fb" message is causing your boot problems, but we can try to work around it. Add this line:
GRUB_GFXPAYLOAD _LINUX= text
to /etc/default/grub, re-run 'sudo update-grub2', and see if you can boot the bisect kernels after that. This should prevent the vesafb driver from trying to use the framebuffer.