i915: wait for fence in prepare_plane_fb

In intel_prepare_plane_fb, if fb is backed by dma-buf, wait for exclusive
fence

v2: First commit
v3: Remove object_name_lock acquire
    Move wait from intel_atomic_commit() to intel_prepare_plane_fb()
v4: Wait only on exclusive fences, interruptible with no timeout
v5: Style tweaks to more closely match rest of file
v6: Properly handle interrupted waits
v7: No change
v8: No change

Link: https://patchwork.kernel.org/patch/7704181/
Signed-off-by: Alex Goins <agoins@nvidia.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>