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Sometimes we want to disable all the screens on a system, because that
will allow the graphics card to be put into low-power states. The
problem is that, for example, while all screens are disabled, if we
get a hotplug interrupt, fbcon will decide to set a mode instead of
keeping everything disabled, which will remove us from our low power
states.

Let's assume that if there's a DRM master, it will be able to do
whatever is appropriate when we get the hotplug.

This problem can be reproduced by the runtime PM test program from
intel-gpu-tools: we disable all the screens so the graphics device can
be put into D3, then something triggers a hotplug interrupt, fbcon
sets a mode and breaks our test suite. The problem can be reproduced
more easily by the "i2c" subtest.

Other approaches considered for the problem:
    - Return "false" if "bound == 0" and the caller of
      drm_fb_helper_is_bound is a hotplug handler. This would break
      the case where the ma...