drm/amdgpu: Use kvmalloc_array for allocating VRAM manager nodes array

commit 6fa39bc1e01dab8b4f54b23e95a181a2ed5a2d38 upstream.

It can be quite big, and there's no need for it to be physically
contiguous. This is less likely to fail under memory pressure (has
actually happened while running piglit).

Cc: stable@vger.kernel.org
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>