drm/amd/display: Add DFS reference clock field

Add to clk_mgr_internal struct, for future use.

Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Reviewed-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>