drm/atomic-helper: Don't skip plane disabling on active CRTC

Since commit "drm/atomic-helper: Add option to update planes only on
active crtc" the drm_atomic_helper_commit_planes() function accepts an
active_only argument to skip updating planes when the associated CRTC is
inactive. Planes being disabled on an active CRTC are incorrectly
considered as associated with an inactive CRTC and are thus skipped,
preventing any plane disabling update from reaching drivers.

Fix it by checking the state of the CRTC stored in the old plane state
for planes being disabled.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>