The new way of doing this does not involve blacklisting any modules. You only have to send two parameters to the kernel:

For Caribbean Islands GPUs

radeon.cik_support=0 amdgpu.cik_support=1

For Sea Islands GPUs

radeon.si_support=0 amdgpu.si_support=1

I personally like to add it as default, so I edit /etc/default/grub, and then I add the kernel parameters to GRUB_CMDLINE_LINUX_DEFAULT, like this:

# If you change this file, run 'update-grub' afterwards to update
# /boot/grub/grub.cfg.
# For full documentation of the options in this file, see:
#   info -f grub -n 'Simple configuration'

GRUB_DEFAULT=0
GRUB_HIDDEN_TIMEOUT=0
GRUB_HIDDEN_TIMEOUT_QUIET=true
GRUB_TIMEOUT=10
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_CMDLINE_LINUX_DEFAULT="radeon.cik_support=0 amdgpu.cik_support=1 radeon.si_support=0 amdgpu.si_support=1"
GRUB_CMDLINE_LINUX=""

# Uncomment to enable BadRAM filtering, modify to suit your needs
# This works with Linux (no patch required) and with any kernel that obtains
# the memory map information from GRUB (GNU Mach, kernel of FreeBSD ...)
#GRUB_BADRAM="0x01234567,0xfefefefe,0x89abcdef,0xefefefef"

# Uncomment to disable graphical terminal (grub-pc only)
#GRUB_TERMINAL=console

# The resolution used on graphical terminal
# note that you can use only modes which your graphic card supports via VBE
# you can see them in real GRUB with the command `vbeinfo'
#GRUB_GFXMODE=640x480

# Uncomment if you don't want GRUB to pass "root=UUID=xxx" parameter to Linux
#GRUB_DISABLE_LINUX_UUID=true

# Uncomment to disable generation of recovery mode menu entries
#GRUB_DISABLE_RECOVERY="true"

# Uncomment to get a beep at grub start
#GRUB_INIT_TUNE="480 440 1"

Remember to run update-grub and reboot the system so the changes take effect.

I can confirm this works for a 390x running Ubuntu since 17.10. Also tried 18.04, 18.10, 19.04, 19.10, 20.04 since I keep using it as they get released. (I believe you can also do the same on 17.04)

Alternatively, if you want to enable the new Dynamic Power Management and Display Code experimental support, you need to also add the following lines

amdgpu.dc=1
amdgpu.dpm=1

For more information regarding DC code check this link: https://wiki.archlinux.org/index.php/AMDGPU#AMD_DC

From my personal experience with a 390x card, DC code was unstable and I had to disable it to get an stable kernel. I haven't retried to enable it in the last couple of months so I don't know if this bug in DC was fixed.