drm/atomic: Paper over locking WARN in default_state_clear

In

commit 6f75cea6
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed Nov 19 18:38:07 2014 +0100

    drm/atomic: Only destroy connector states with connection mutex held

I tried to fix races of atomic commits against connector
hot-unplugging. The idea is to ensure lifetimes by holding the
connection_mutex long enough. That works for synchronous commits, but
not for async ones.

For async atomic commit we really need to fix up connector lifetimes
for real. But that's a much bigger task, so just add more duct-tape:
For cleaning up connector states we currently don't need the connector
itself. So NULL it out and remove the locking check. Of course that
check was to protect the entire sequence, but the modeset itself
should be save since currently DP MST hot-removal does a dpms-off. And
that should synchronize with any outstanding async atomic commit.

Or at least that's my hope, this is all a giant mess.

Reported-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>