Somewhere between c93efb0a4f and 1edc53a66b a regression was introduced,
causing SDDM launching into a pure white screen instead of the login screen.
There's nothing logged to either /var/log/Xorg.0.log or /var/log/sddm.log and
dmesg is also clear. Downgrading to c93efb0a4f lets me login again.

I have not bisected yet, but maybe this information is already enough to find
the cause. For the full stack affected by this regression, please see below.

GPU: Hawaii PRO [Radeon R9 290] (ChipID = 0x67b1)
Mesa: Git:master/1edc53a66b + the modified version of attachment 127812 [details] [review]
libdrm: 2.4.74-1
LLVM: SVN:trunk/r291276 (4.0 devel) +
<https://reviews.llvm.org/D26348?download=true>
X.Org: 2:1.19.0-3
Linux: 4.9.1
Firmware: 20161130-2
libclc: Git:master/b906699f68
DDX (amdgpu): 1.2.0-1+b1

Let me know, if you need anything else (besides a bisect).