drm/atomic_helper: Add drm_atomic_helper_disable_planes_on_crtc()

Add drm_atomic_helper_disable_planes_on_crtc() for disabling all
planes associated with the given CRTC. This can be used for instance
in the CRTC helper disable callback to disable all planes before
shutting down the display pipeline.

v2:
- Address Daniels review comments [1]
  - Do atomic_begin() and atomic_flush() always if they are defined and
    atomic knob is set
  - update kerneldoc
- Put drm_atomic_helper_disable_planes_on_crtc() after
  drm_atomic_helper_commit_planes_on_crtc() in drm_atomic_helper.c
  to have functions in the same order as in drm_atomic_helper.h

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448633641-6486-1-git-send-email-jsarha@ti.com


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>