drm/amd/display: move FPU code on dcn21 clk_mgr

The -mno-gnu-attribute option in dcn21 clk mgr makefile hides a soft vs
hard fp error for powerpc. After removing this flag, we can see some FPU
code remains there:

/gcc-11.3.0-nolibc/powerpc64-linux/bin/powerpc64-linux-ld:
drivers/gpu/drm/amd/amdgpu/../display/dc/dml/display_mode_lib.o uses
hard float,
drivers/gpu/drm/amd/amdgpu/../display/dc/clk_mgr/dcn21/rn_clk_mgr.o uses
soft float

Therefore, remove the -mno-gnu-attribute flag for dcn21/powerpc and move
FPU-associated code to DML folder.

Signed-off-by: Melissa Wen <mwen@igalia.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>