drm/amdgpu: use RREG32_KIQ to read register when get cg state

Use RREG32_KIQ to read gfx register when try to get gfx/sdma
clockgating state instead of RREG32, as it will result
to system hard hang when GPU is enter into GFXOFF state.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>