Bisecting results are below.

This is an Asus model STRIX-R9390-DC3OC-8GD5-GAMING.

Don't think is relevant, oter than why someone was looking at this code, but
this commit immediately trails a bunch of the vkms commits, and the crash/to
black screen happens right when amdgpu is attempting to automatically switch to
kms.

0d99889109892396a8164bf6dd178e36d3fe3166 is the first bad commit
commit 0d99889109892396a8164bf6dd178e36d3fe3166
Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
Date:   Thu Jun 28 16:13:07 2018 +0300

    drm/fb-helper: Eliminate the .best_encoder() usage

    Instead of using the .best_encoder() hook to figure out whether a given
    connector+crtc combo will work, let's instead do what userspace does and
    just iterate over all the encoders for the connector, and then check
    each crtc against each encoder's possible_crtcs bitmask.

    v2: Avoid oopsing on NULL encoders (Daniel)
        s/connector_crtc_ok/connector_has_possible_crtc/

    Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
    Cc: Harry Wentland <harry.wentland@amd.com>
    Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
    Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
    Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
    Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
    Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
    Link:
https://patchwork.freedesktop.org/patch/msgid/20180628131315.14156-2-ville.syrjala@linux.intel.com