drm/i915: use new iterator in i915_gem_object_wait_priority

Simplifying the code a bit.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211116102431.198905-3-christian.koenig@amd.com