drm/i915: Wait for object idle without locks in atomic_commit, v2.

Make pinning and waiting a separate step, and wait for object idle
without struct_mutex held.

Changes since v1:
- Do not wait when a reset is in progress.
- Remove call to i915_gem_object_wait_rendering for
  intel_overlay_do_put_image (Chris Wilson)

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>