drm/atomic-helper: WARN if legacy plane fb pointers are bogus when committing duplicated state

drm_atomic_helper_commit_duplicated_state() should only be called
resume/reset/load_detect paths where plane->old_fb should always be
NULL and plane->fb should be equal to the new_plane_state->fb.
Assert that is indeed the case.

Cc: martin.peres@free.fr
Cc: chris@chris-wilson.co.uk
Cc: Dave Airlie <airlied@gmail.com> (v1)
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180322152313.6561-5-ville.syrjala@linux.intel.com


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