drm/amdgpu/gfx10: refine mgcg setting

1. enable ENABLE_CGTS_LEGACY to fix specviewperf11 random hang.
2. remove obsolete RLC_CGTT_SCLK_OVERRIDE workaround.

Signed-off-by: Jiansong Chen <Jiansong.Chen@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org