drm/atomic-helper: Again check modeset *before* plane states

This essentially reverts

commit 934ce1c2
Author: Rob Clark <robdclark@gmail.com>
Date:   Wed Nov 19 16:41:33 2014 -0500

    drm/atomic: check mode_changed *after* atomic_check

Depending upon the driver both orders (or maybe even interleaving) is
required:
- If ->atomic_check updates ->mode_changed then helper_check_modeset
  must be run afters.
- If ->atomic_check depends upon accurate adjusted dotclock values for
  e.g. watermarks, then helper_check_modeset must be run first.

The failure mode in the first case is usually a totally angry hw
because the pixel format switching doesn't happen. The failure mode in
the later case is usually nothing, since in most cases the old
adjusted mode from the previous modeset wont be too far off to be a
problem. So just underruns and perhaps even just suboptimal (from a
power consumption) watermarks.

Furthermore in the transitional helpers we only call ->atomic_check
after the new modeset state has been fully set up (and hence
computed).

Given that asymmetry in expected failure modes I think it's safer to
go back to the older order. So do that and give msm a special check
function to compensate.

Also update kerneldoc to explain this a bit.

v2: Actually add the missing hunk Rob spotted.

v3: Move msm_atomic_check into msm_atomic.c, requested by Rob.

Cc: Rob Clark <robdclark@gmail.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Tested-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>