drm/atomic: Update comment to match the code

The kerneldoc for drm_atomic_crtc_needs_modeset() is outdated and no
longer reflects the actual code. Fix that up to remove confusion.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170731091343.21363-1-thierry.reding@gmail.com