I found a way to enable the amdgpu support for this hardware in the
standard LTS kernel!  Based on this comment on a YouTube video reviewing
this laptop:

  https://www.youtube.com/watch?v=FjegdWtHuOE&lc=UgzbtkQw4vuk6ls-
6vF4AaABAg

Add "amdgpu.exp_hw_support=1" to the kernel command line in /etc/default/grub, 
run
update-grub, and reboot.  This FIXES ALL THE ISSUES that I encountered in this 
bug.  I am running linux-image-5.4.0-42-generic:amd64 (5.4.0-42.46) which is 
the current LTS kernel.

The same poster also recommended turning on this Firefox about:config
setting, to avoid screen tearing while playing YouTube videos:
layers.acceleration.force-enabled.

I have seen reports (comments below that same YouTube video, and
elsewhere) that there are small kernel patches in the 5.7 kernels that
also fix the screen brightness settings on this hardware.  Perhaps a
future Ubuntu kernel update can cherry-pick those fixes, and
automatically enable the experimental hardware support for this
integrated AMD Ryzen 5 4500U CPU/GPU?  Would be good for these fixes to
get into the Ubuntu 20.04.1 point release, too; these laptops are very
fast and cost $600 so they are selling very well.

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https://bugs.launchpad.net/bugs/1888088

Title:
  [amdgpu] [Lenovo IdeaPad Flex 5 14ARE05] Display won't rotate, nor do
  fractional scaling

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