After I had to upgrade from FreeBSD-11 to FreeBSD-12 (since new Qt5 now do not work with base OpenSSL, and some other problems), I migrated from graphics/drm-fbsd11.2-kmod to graphics/drm-fbsd12.0-kmod obviously.
May be it is a wrong port I am complaining, but there was no such behaviour previously (on -11 branch).
When it've loaded amdkms.ko from rc.conf line, I get the error periodicaly on console (no X-server loaded yet):
[drm:radeon_vga_detect] VGA-1: probed monitor but no|invalid EDID
and my poor old monitor blinks which is not good for it or my eyes.
I suppose it is something to do with my upgrade and drm-fbsd12.0-kmod port, since it was not with 11.2 version.
And yes, under Xorg there appears no problem. In Xorg.0.log it quite nicely recognizes my monitor and its EDID, makes good refresh rate and no blinks.
So, it is problem for me in vt console after amdkms.ko load only.