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

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>
Cc: stable@vger.kernel.org