I'm still having this issue.
Here's some dmesg output:

[  363.951100] -- 2560x1440 --
[  363.951101] radeon_crtc->hw_mode.crtc_htotal: 2640
[  363.951101] radeon_crtc->hw_mode.clock: 482640
[  363.951102] radeon_crtc->hw_mode.crtc_vblank_end: 1525
[  363.951102] radeon_crtc->hw_mode.crtc_vdisplay: 1440
[  363.951103] radeon_crtc->v_border: 0
[  363.951103] vblank_time_us: 425
[  363.951104] vblank_lines: 85
[  363.951104] line_time_us: 5
[  363.951105] vblank_time: 425, switch_limit: 450