Since Linux 3.17.3 calling $startx will freeze or reboot my system.
I have isolated the following commit as the cause of my problem:

commit ffe0245532b98efc4bc0e06f29c51d3f0e471152
Author: Alex Deucher <alexander.deucher@amd.com>
Date:   Mon Oct 13 11:51:50 2014 -0400

    drm/radeon: fix speaker allocation setup
    
    commit 4910403836ded89803fab201d4b5caaa85de3a89 upstream.
    
    If the sad_count is 0, set the hw to stereo and change
    the error message to a warn.  A lot of monitors don't
    set the speaker allocation block.
    
    Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Reverting this brings things back to normal.

My card:
01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Oland XT [Radeon HD 8670 / R7 250]
01:00.1 Audio device: Advanced Micro Devices, Inc. [AMD/ATI] Cape Verde/Pitcairn HDMI Audio [Radeon HD 7700/7800 Series]

Screen:
Samsung S22D300HY (via HDMI)

OS:
Archlinux up-2-date... Mesa 10.3.3 + patch from bug 85647, xorg-server 1.16.2, xf86-video-ati 7.5.0

Can provide additional infos if needed.

Thanks!