The -modesetting ddx has a totally broken idea of how atomic works:
- doesn't disable old connectors, assuming they get auto-disable like
  with the legacy setcrtc
- assumes ASYNC_FLIP is wired through for the atomic ioctl
- not a single call to TEST_ONLY

Iow the implementation is a 1:1 translation of legacy ioctls to
atomic, which is a) broken b) pointless.

We already have bugs in both i915 and amdgpu-DC where this prevents us
from enabling neat features.

If anyone ever cares about atomic in X we can easily add a new atomic
level (req->value == 2) for X to get back the shiny toys.

Since these broken versions of -modesetting have been shipping,
there's really no other way to get out of this bind.

v2:
- add an informational dmesg output (Rob, Ajax)
- reorder after the DRIVER_ATOMIC check to avoid useless noise (Ilia)
- allow req->value > 2 so that X can do another attempt at atomic in
  the future

v3: Go with paranoid, insist that the X should be first (suggested by
Rob)

Cc: Ilia Mirkin <imirkin@alum.mit.edu>
References: https://gitlab.freedesktop.org/xorg/xserver/issues/629
References: https://gitlab.freedesktop.org/xorg/xserver/merge_requests/180
References: abbc0697 ("drm/fb: revert the i915 Actually configure untiled displays from master")
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> (v1)
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com> (v1)
Cc: Michel Dänzer <michel@daenzer.net>
Cc: Alex Deucher <alexdeucher@gmail.com>
Cc: Adam Jackson <ajax@redhat.com>
Acked-by: Adam Jackson <ajax@redhat.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Rob Clark <robdclark@gmail.com>
Acked-by: Rob Clark <robdclark@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190905185318.31363-1-daniel.vetter@ffwll.ch