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 <thomas.hellstrom@linux.intel.com>
Fixes: 2f6b90da ("drm/i915: Use vma resources for async unbinding")
Cc: <stable@vger.kernel.org> # v5.18+
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221223092011.11657-1-nirmoy.das@intel.com

drivers/gpu/drm/i915/i915_vma.c

@@ -2149,7 +2149,7 @@ int i915_vma_unbind_async(struct i915_vma *vma, bool trylock_vm)
 	if (!obj->mm.rsgt)
 		return -EBUSY;
 
-	err = dma_resv_reserve_fences(obj->base.resv, 1);
+	err = dma_resv_reserve_fences(obj->base.resv, 2);
 	if (err)
 		return -EBUSY;