Avoid holding locks during hotplug callback

... as SurfaceFlinger could still be calling into Ranchu HWC on
another thread while Ranchu HWC is calling into SurfaceFlinger
with the register hotplug callback.

Effectively the same deadlock detected with aosp/2427265 but
at the lower DRM level.

Bug: b/269515370
Test: atest CuttlefishDisplayHotplugTest (a couple times)
Change-Id: I58312ac5e5de6dea2aa260f0ef49b13da1442ad1
1 file changed