If you have not already done so, try disabling the watchdog timer:

MODULE_PARM_DESC(lockup_timeout, "GPU lockup timeout in ms (default 10000 = 10
seconds, 0 = disable)");
module_param_named(lockup_timeout, radeon_lockup_timeout, int, 0444);

As part of HSA/ROC development we dropped the priority of compute work relative
to graphics which improved interactivity and *almost* eliminated timeouts
without having to disable the timer  - when I get back in the office I'll dig
up the changes. In the meantime, I think disabling the timer will do what you
need although you will still have sluggish graphics while long-running kernels
are active.

Lowering the priority of compute waves across the board won't be a fully
general solution because there are going to be some cases (eg Valve's recent
work with using high priority compute to improve VR smoothness) where compute
will need to be *higher* priority than graphics but it should cover most cases
other than "simultaneously running GROMACS and VR".