im using amd-staging-4.12 branch from
https://cgit.freedesktop.org/~agd5f/linux/

gist of dmesg
https://gist.github.com/gulafaran/af0edfff6dabafe12f451a2caaa5bd58

tried on mesa 17.2 release and 
17.3.0_devel.95819.9aba643e3c-1

and tried both llvm 5.0.0 and
6.0.0svn_r313351-1

03:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Vega
[Radeon RX Vega] (rev c3)

steam doesnt render quite correctly as can be seen here
https://i.imgur.com/gRxuOz8.png , no particular error is printed to stdout or
stderr nor dmesg. ive tried various games and benchmarks which all works just
fine. its only steams GUI thats misbehaving so far.

no change running it with STEAM_RUNTIME=0 or 1, same goes with
STEAM_RUNTIME_PREFER_HOST_LIBRARIES=0 or 1 .

and this happend when i switched from nvidia card to the vega 56. so the steam
install should be fine as it has been working on nvidia blob and nouveau, even
tried removing and recreating the ~/ steam files.