drm/i915: Defer modeset cleanup to a secondary task

drm/i915: Defer modeset cleanup to a secondary task

If we avoid cleaning up the old state immediately in
intel_atomic_commit_tail() and defer it to a second task, we can avoid
taking heavily contended locks when the caller is ready to procede.
Subsequent modesets will wait for the cleanup operation (either directly
via the ordered modeset wq or indirectly through the atomic helperr)
which keeps the number of inflight cleanup tasks in check.

As an example, during reset an immediate modeset is performed to disable
the displays before the HW is reset, which must avoid struct_mutex to
avoid recursion. Moving the cleanup to a separate task, defers acquiring
the struct_mutex to after the GPU is running again, allowing it to
complete. Even in a few patches time (optimist!) when we no longer
require struct_mutex to unpin the framebuffers, it will still be good
practice to minimise the number of contention points along reset. The
mutex dependency still exists (as one modeset flushes the other), but in
the short term it resolves the deadlock for simple reset cases.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101600
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180623103951.23889-1-chris@chris-wilson.co.uk
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>