drm/atomic: add all affected planes in drm_atomic_helper_check_modeset

Drivers may need to recalculate plane state when a modeset occurs,
not reliably adding them might cause hard to debug bugs.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>