drm/core: Fix old_fb handling in drm_mode_atomic_ioctl.

plane_mask should be cleared inside the retry loop, because it gets
reset on every retry. Without this fix the plane->fb refcounting might
get out of sync on retries, resulting in either leaked memory or
use-after-free.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: stable@vger.kernel.org #v4.3
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1447237751-9663-3-git-send-email-maarten.lankhorst@ubuntu.com