drm: Fix comment making reference to non-existing function

Documentation for drm_atomic_crtc_for_each_plane() makes reference to
a function called drm_crtc_for_each_pending_plane(). I've guessed that
the actual function name is drm_atomic_crtc_state_for_each_plane() as
that matches best the intent of the comment.

Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1465571005-3877-1-git-send-email-Liviu.Dudau@arm.com