drm/amdgpu/cgs: always set reference clock in mode_info

It's relevent regardless of whether there are displays
enabled.  Fixes garbage values for ref clock in powerplay
leading to incorrect fan speed reporting when displays
are disabled.

bug: https://bugs.freedesktop.org/show_bug.cgi?id=101653


Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org