(In reply to Nicolas Koch from comment #4)
> Forgot to mention, of course I ran glxinfo with DRI_PRIME=1

Same problem here with ThinkPad L450

05:00.0 Display controller: Advanced Micro Devices, Inc. [AMD/ATI] Jet XT
[Radeon R5 M240] (rev ff)

I found than setting both integrated and discrete gpu to DRI3 allows to run
glxgears, however it hard-freezes once any gpu intensive application is run for
more than few minutes. Using additional vblank_mode=0 causes this hard-freeze
to appear faster. Before hard-freeze there is massive fps drop during which it
is possible to kill the application and avoid freeze.

Also using

echo "DDIS" > /sys/kernel/debug/vgaswitcheroo/switch

and restarting X causes black screen (with backlight turned of) but that might
be another bug