drm/amdgpu: correct reference clock value on vega10

commit 76d6172b6fab16455af4b67bb18a3f66011592f8 upstream.

Old value from bringup was wrong.

Signed-off-by: Ken Wang <Ken.Wang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>