I arrived here by way of having a similar problem and bisecting it down to the
very same commit. When running a kernel containing it, on my R9 285,
modesetting on my two DVI monitors takes very long and results in a weird
resolution that looks like 1920x1080 to the system and 960x1080 to the actual
device (if its info menu can be trusted). Most other resolutions I tried fail
with out of range.