drm/amdgpu/atom: fix ps allocation size for EnableDispPowerGating

commit 05b4017b37f1fce4b7185f138126dd8decdb381f upstream.

We were using the wrong structure which lead to an overflow
on some boards.

bug: https://bugs.freedesktop.org/show_bug.cgi?id=101387


Acked-by: Chunming Zhou <david1.zhou@amd.com>
Acked-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>