drm_atomic_helper_shutdown() needs to release the reference held by
plane->fb. Since commit 49d70aea ("drm/atomic-helper: Fix leak in
disable_all") we're doing that by calling drm_atomic_clean_old_fb() in
drm_atomic_helper_disable_all(). This also leaves plane->fb == NULL
afterwards. However, since drm_atomic_helper_disable_all() is also
used by the i915 gpu reset code
drm_atomic_helper_commit_duplicated_state() then has to undo the
damage and put the correct plane->fb pointers back in (and also
adjust the ref counts to match again as well).

That approach doesn't work so well for load detection as nothing
sets up the plane->old_fb pointers for us. This causes us to
leak an extra reference for each plane->fb when
drm_atomic_helper_commit_duplicated_state() calls
drm_atomic_clean_old_fb() after load detection.

To fix this let's call drm_atomic_clean_old_fb() only for
drm_atomic_helper_shutdown() as that's the only time we need to
actually drop the plane->fb references. In all the other cases
(load detection, gpu reset) we want to leave plane->fb alone.

v2: Don't inflict the clean_old_fbs bool to drivers (Daniel)
v3: Squash in the revert and rewrite the commit msg (Daniel)

Cc: martin.peres@free.fr
Cc: chris@chris-wilson.co.uk
Cc: Dave Airlie <airlied@gmail.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180322152313.6561-3-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> #pre-squash
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>