drm/amd/display: remove header from source file

Since "rn_clk_mgr.h" needs ‘struct clk_mgr_internal’ which is declared
in "clk_mgr_internal.h", include "clk_mgr_internal.h" in "rn_clk_mgr.h"
instead of in its source file.
Because of the change above, change the order of '#include
"rn_clk_mgr.h"', so that the necessary structs are visible to
dcn20_clk_mgr.h.

Signed-off-by: Magali Lemes <magalilemes00@gmail.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Reviewed-by: Maíra Canal <mairacanal@riseup.net>
Reviewed-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>