drm/amd/display: make some rn_clk_mgr structs and funcs static

[Why]
There are some structures and functions meant only to be used in the
scope of that single rn_clk_mgr c file.

[How]
Make structs and funcs static if only meant to be used within
rn_clk_mgr

Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>