drm/i915/gt: Fix a double free in gen8_preallocate_top_level_pdp

Our code analyzer reported a double free bug.

In gen8_preallocate_top_level_pdp, pde and pde->pt.base are allocated
via alloc_pd(vm) with one reference. If pin_pt_dma() failed, pde->pt.base
is freed by i915_gem_object_put() with a reference dropped. Then free_pd
calls free_px() defined in intel_ppgtt.c, which calls i915_gem_object_put()
to put pde->pt.base again.

As pde->pt.base is protected by refcount, so the second put will not free
pde->pt.base actually. But, maybe it is better to remove the first put?

Fixes: 82adf901 ("drm/i915/gt: Shrink i915_page_directory's slab bucket")
Signed-off-by: Lv Yunlong <lyl2019@mail.ustc.edu.cn>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210426124340.4238-1-lyl2019@mail.ustc.edu.cn