drm/amdgpu/gfx10: disable gfxoff when reading rlc clock

Otherwise we readback all ones.  Fixes rlc counter
readback while gfxoff is active.

Reviewed-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org