drm/atomic-helper: debug output for modesets

With the combination of ->enable and ->active it's a bit complicated
to follow what exactly is going on sometimes within a full modeset.
Add debug output to make this all traceable.

Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

diff --cc drivers/gpu/drm/drm_atomic_helper.c
Simple merge