drm/i915: use the new iterator in i915_sw_fence_await_reservation v3

Simplifying the code a bit.

v2: use dma_resv_for_each_fence instead, according to Tvrtko the lock is
    held here anyway.
v3: back to using dma_resv_for_each_fence_unlocked.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211116102431.198905-4-christian.koenig@amd.com