drm: use new iterator in drm_gem_fence_array_add_implicit v3

Simplifying the code a bit.

v2: add missing rcu_read_lock()/unlock()
v3: switch to locked version

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211005113742.1101-24-christian.koenig@amd.com