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

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>