drm/i915: Remove redundant i915_request_await_object in blit clears

One i915_request_await_object is enough and we keep the one under the
object lock so it is final.

At the same time move async clflushing setup under the same locked
section and consolidate common code into a helper function.

v2:
 * Emit initial breadcrumbs after aways are set up. (Chris)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michael J. Ruhl <michael.j.ruhl@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200615151449.32605-1-tvrtko.ursulin@linux.intel.com