drm/amd/amdgpu: Enabling Power Gating for Stoney platform

Removed condition checks to skip the power gating feature for
stoney platform.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Vijendar Mukunda <vijendar.mukunda@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>