drm/i915: Redo "Remove i915_request.lock requirement for execution callbacks"

The reordering and rebasing of commit 2e4c6c1a ("drm/i915: Remove
i915_request.lock requirement for execution callbacks") caused it to
revert an earlier correction. Let us restore commit 99f0a640d464
("drm/i915: Remove requirement for holding i915_request.lock for
breadcrumbs")

Fixes: 2e4c6c1a

 ("drm/i915: Remove i915_request.lock requirement for execution callbacks")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200925101107.27869-1-chris@chris-wilson.co.uk