drm/amdgpu: Remove redundant code in gfx_v8_0.c

the CG related registers have been programed in golden setting
PG register default value is 0.

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Hang Zhou <hang.zhou@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>