drm/amd/display: move clk_mgr files to right place

[Why]
Better organization

[How]
Move clk_mgr files under dc/clk_mgr

Signed-off-by: Eric Yang <Eric.Yang2@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>