amdgpu: Fix pointer/integer mismatch warning

amdgpu: Fix pointer/integer mismatch warning

On 32-bit:

    ../amdgpu/amdgpu_bo.c: In function ‘amdgpu_find_bo_by_cpu_mapping’:
    ../amdgpu/amdgpu_bo.c:554:13: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
           cpu < (void*)((uintptr_t)bo->cpu_ptr + bo->alloc_size))
                 ^

Indeed, as amdgpu_bo_info.alloc_size is "uint64_t", the sum is
always 64-bit, while "void *" can be 32-bit or 64-bit.

Fix this by casting bo->alloc_size to "size_t", which is either
32-bit or 64-bit, just like "void *".

Fixes: c6493f360e7529c2 ("amdgpu: Eliminate void* arithmetic in amdgpu_find_bo_by_cpu_mapping")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
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v2:
  - Add Reviewed-by.