drm/nouveau: Handle fbcon suspend/resume in seperate worker

[ Upstream commit 15266ae38fe09dae07bd8812cb7a7717b1e1d992 ]

Resuming from RPM can happen while already holding
dev->mode_config.mutex. This means we can't actually handle fbcon in
any RPM resume workers, since restoring fbcon requires grabbing
dev->mode_config.mutex again. So move the fbcon suspend/resume code into
it's own worker, and rely on that instead to avoid deadlocking.

This fixes more deadlocks for runtime suspending the GPU on the ThinkPad
W541. Reproduction recipe:

 - Get a machine with both optimus and a nvidia card with connectors
   attached to it
 - Wait for the nvidia GPU to suspend
 - Attempt to manually reprobe any of the connectors on the nvidia GPU
   using sysfs
 - *deadlock*

[airlied: use READ_ONCE to address Hans's comment]

Signed-off-by: Lyude <lyude@redhat.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Kilian Singer <kilian.singer@quantumtechnology.info>
Cc: Lukas Wunner <lukas@wunner.de>
Cc: David Airlie <airlied@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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