drm/atomic: add commit_planes_on_crtc helper

drm_atomic_helper_commit_planes calls all atomic_begin's first,
then updates all planes, finally calling atomic_flush.

Some drivers may want to things like disabling irq's
from their atomic_begin, in which case a second call to atomic_begin
will splat. By using commit_planes_on_crtc on each crtc in the
atomic state they'll evade that issue.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
[danvet: Extend kerneldoc a bit as discussed with Maarten on irc.]
[danvet: Squash in fixup to check for crtc_funcs in all places.
Reported by Dan Carpenter.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>