drm/atomic: Add drm_atomic_crtc_state_for_each_plane_state

... and use it in msm&vc4. Again just want to encapsulate
drm_atomic_state internals a bit.

The const threading is a bit awkward in vc4 since C sucks, but I still
think it's worth to enforce this. Eventually I want to make all the
obj->state pointers const too, but that's a lot more work ...

v2: Provide safe macro to wrap up the unsafe helper better, suggested
by Maarten.

v3: Fixup subject (Maarten) and spelling fixes (Eric Engestrom).

Cc: Eric Anholt <eric@anholt.net>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464877304-4213-1-git-send-email-daniel.vetter@ffwll.ch