drm/fb-helper: Give up on kgdb for atomic drivers

It just doesn't work. It probably stopped working way, way before that
(e.g. i915 grabbed random mutexes all over in modeset code at least
since gen6), but with atomic and all the ww_mutex stuff it's indeed
hopeless.

Remove ->mode_set_base_atomic from the 2 atomic drivers (i915 and
nouveau) that still had one (both had dummy implementations already
anyway), and shunt atomic drivers in the helpers debug_enter/leave
functions.

I'll leave the code in for radeon and amdgpu, but I think as soon as
amdgpu is atomic we should think about just ripping it out. Only
having it around for radeon and pre-nv50 is rather pointless. This
would also allow us to nuke all that code from fbdev.

Funny part is that _all_ kms drivers set this hook, despite that no
one else provides the required ->mode_set_base_atomic implementation.

The reason I'm jumping on this is that I want to wire up a full
acquire ctx for the benefit of atomic drivers, everywhere. And the
debug_enter/leave implementations call ->gamma_set. And there's just
no way ever we can create an acquire_ctx in the nmi context of kgdb.

Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170403083304.9083-11-daniel.vetter@ffwll.ch