drm/atomic: clarify the rules around drm_atomic_state->allow_modesetHEADstuff

msm is automagically upgrading normal commits to full modesets, and
that's a big no-no:

- for one this results in full on->off->on transitions on all these
  crtc, at least if you're using the usual helpers. Which seems to be
  the case, and is breaking uapi

- further even if the ctm change itself would not result in flicker,
  this can hide modesets for other reasons. Which again breaks the
  uapi

v2: I forgot the case of adding unrelated crtc state. Add that case
and link to the existing kerneldoc explainers. This has come up in an
irc discussion with Manasi and Ville about intel's bigjoiner mode.
Also cc everyone involved in the msm irc discussion, more people
joined after I sent out v1.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Simon Ser <contact@emersion.fr>
Cc: Manasi Navare <navaremanasi@google.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Abhinav Kumar <quic_abhinavk@quicinc.com>
Cc: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

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