Try checking the actual access permissions for /dev/fb0 and /dev/vchiq. You may need to adjust the latter by doing something like
echo 'SUBSYSTEM=="vchiq",GROUP="video",MODE="0660"' > /etc/udev/rules.d/10-vchiq-permissions.rules
and double-checking that any unprivileged account used to run Mathematica is a member of the video group.