drm/atomic: Update old_fb after setting a property.
drm/atomic: Update old_fb after setting a property.

This change updates the old_fb pointer only after acquiring the plane lock,
if there are no properties the fb cannot have been changed either, so
this works out correctly.

Found in a discussion with Rob Clark.

Cc: Rob Clark <915c10c999604870200b6defbe633d857c856ca0@gmail.com>
Signed-off-by: Maarten Lankhorst <743c2818a341d71b99ed2b566f9f80c2fc288f93@linux.intel.com>
Reviewed-by: Rob Clark <915c10c999604870200b6defbe633d857c856ca0@gmail.com>
Signed-off-by: Daniel Vetter <c1b6782c4af8f0673da8923a0702a1832e5940f4@ffwll.ch>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs