drm/amdgpu/st: move ATC CG golden init from gfx to mc

It's technically an MC register so make sure we initialize it
in the MC module rather than the gfx module.  Since other bits
in the same register are used to enable ATC CG features make
sure we apply the golden setting first.

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