drm/amdgpu: disable coarse grain clockgating for ST

The CGCG feature  on Stoney is causing GFX related
issues such as freezes and blank outs.

Signed-off-by: Shirish S <shirish.s@amd.com>
Reviewed-by: Arindam Nath <arindam.nath@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>