amdgpu/amdgpu_ids: fix kmalloc_array not uses number as first arg

Fix check_patch.pl warning:
kmalloc_array uses number as first arg, sizeof is generally wrong.
+fences = kmalloc_array(sizeof(void *), id_mgr->num_ids,
GFP_KERNEL);

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Bernard Zhao <bernard@vivo.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>