drm_hwcomposer: Rework HwcDisplay disposal to avoid races

The code prior to this commit has a flaw:

    HwcDisplay::~HwcDisplay() {
      ...
      auto &main_lock = hwc2_->GetResMan().GetMainLock();
      /* Unlock to allow pending vsync callbacks to finish */
      main_lock.unlock();

At this point display is no longer in displays_[] list. After
lock is released, hwc2 API thread starts to process transactions
which may fail with BAD_SIAPLAY responce and cause SF to crash.

      vsync_worker_.VSyncControl(false);
      vsync_worker_.Exit();
      main_lock.lock();
    }

1. Rework the logic in order to avoid such scenariuos:
1.a. Temporary switch non-primary unplugged displays to headless state
     allowing remaining transactions to succeed without impacting the
     pipeline.
1.b. Give 100mSec delay before destroying / removing display from the
     displays_[] list to allow all pending hwc2 transactions to complete.

2. Support hotswap of the DrmDisplayPipeline, which makes primary display
   reattaching process smoother.
   Now SF should be able to gracefully remove all layers.

Signed-off-by: Roman Stratiienko <roman.o.stratiienko@globallogic.com>
2 jobs for fix-race-during-hotplug in 17 minutes and 57 seconds (queued for 3 seconds)