I think I have a similar problem in R9 280X.
My lockups don't result in a kernel panic and the system simply stops, but I
also have the "ring 0/3 stalled for more than Xmsec" messages prior to the
lockup.
Eventually I also end up with the following messages:

[drm:atom_op_jump [radeon]] *ERROR* atombios stuck in loop for more than 5secs
aborting
[drm:atom_execute_table_locked [radeon]] *ERROR* atombios stuck executing CFB6
(len 62, WS 0, PS 0) @ 0xCFD2
[drm:atom_op_jump [radeon]] *ERROR* atombios stuck in loop for more than 5secs
aborting
[drm:atom_execute_table_locked [radeon]] *ERROR* atombios stuck executing CFB6
(len 62, WS 0, PS 0) @ 0xCFD2

Please ask for more information and I'll abide.

Note: currently on Linux 4.13 and using latest Padoka mesa packages.