drm/i915: Reserve enough fence slot for i915_vma_unbind_async

A nested dma_resv_reserve_fences(1) will not reserve slot from the
2nd call onwards and folowing dma_resv_add_fence() might hit the
"BUG_ON(fobj->num_fences >= fobj->max_fences)" check.

I915 hit above nested dma_resv case in ttm_bo_handle_move_mem() with
async unbind:

dma_resv_reserve_fences() from --> ttm_bo_handle_move_mem()
        dma_resv_reserve_fences() from --> i915_vma_unbind_async()
        dma_resv_add_fence() from --> i915_vma_unbind_async()
dma_resv_add_fence() from -->ttm_bo_move_accel_cleanup()

Resolve this by adding an extra fence in i915_vma_unbind_async().

Suggested-by: Thomas Hellström <[email protected]>
Fixes: 2f6b90da9192 ("drm/i915: Use vma resources for async unbinding")
Cc: <[email protected]> # v5.18+
Signed-off-by: Nirmoy Das <[email protected]>
Reviewed-by: Matthew Auld <[email protected]>
Signed-off-by: Matthew Auld <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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