Hi,

I'm getting the same or similar error:
 * Black screen, no plymouth
 * "Cannot find any crtc or sizes" error in dmesg
 * amdgpu.dc=0 works around the issue

...and I might have a lead on it. The regression is between 4.15rc2 and 4.15rc3
(found by downloading kernels from
http://kernel.ubuntu.com/~kernel-ppa/mainline/). 

The only one commit I could see being relevant between 4.15rc2 and 4.15rc3 is
this one:

commit a703c55004e1c5076d57e43771b3e11117796ea0
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date: Mon Dec 4 21:48:18 2017 +0100

    drm: safely free connectors from connector_iter

...it seems relevant because the error I get is "Cannot find any crtc or
sizes", and the code emitting this error message (in
drivers/gpu/drm/drm_fb_helper.c) deals with connectors.

So could this commit cause connectors to be freed where the previous code did
not? It seems like this could be the case, but only if (obj->free_cb == NULL) -
drm_connector_put checks for a obj->free_cb being null whereas
drm_connector_put_safe decrements a refcount regardless of whether a free_cb is
present or not.

Does all this make sense, or am I out on deep water here?