Yeah, you're modifying the wrong file.
debian@beaglebone:~/nfs$ cat /boot/uEnv.txt
#Docs:
http://elinux.org/Beagleboard:U-boot_partitioning_layout_2.0uname_r=4.4.12-ti-r31
#uuid=
#dtb=
##BeagleBone Black/Green dtb's for v4.1.x (BeagleBone White just works..)
##BeagleBone Black: HDMI (Audio/Video) disabled:
dtb=am335x-boneblack-emmc-overlay.dtb
. . .
dtb=am335x-boneblack-emmc-overlay.dtb is the line you're after, and it probably is proceeded by a pound( # ) symbol. So it's commented out. SImply remove the # to uncomment, and then reboot the board.