drm/i915: use the new iterator in i915_request_await_object v2

Simplifying the code a bit.

v2: add missing rcu_read_lock()/rcu_read_unlock()
v3: use dma_resv_for_each_fence instead

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211005113742.1101-20-christian.koenig@amd.com