2019.1:
From 2019.1 release,to select fbdev, Just select libmali-xlnx package from 'petalinux-config -c rootfs' and add the following lines to <plnx-proj-root>/project-spec/meta-user/conf/petalinuxbsp.conf
MALI_BACKEND_DEFAULT = "fbdev"
2018.3:
Up to 2018.3, in order to have Mali fbdev backend, please add following lines to <plnx-proj-root>/project-spec/meta-user/conf/petalinuxbsp.conf
DISTRO_FEATURES_remove = "x11"