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Thanks to apple gpu mux fail we detect an eDP output, but can't read
anything over dp aux. In the resulting failure path we then hit a
paranoid WARN about potential locking.

Since the WARN is pretty useful for normal operation just paper over
it in the failure case by grabbing the demanded (but for init/teardown
not really required) lock.

I've checked our driver unload code and we already don't hold the kms
lock when calling drm_mode_config_cleanup. So this won't lead to a new
deadlock when reloading i915.ko.

v2: Make it compile.
Reported-by: Dave Airlie <airlied@gmail.com>
Cc: Dave Airlie <airlied@gmail.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>