drm/amd/display: Re-arrange FPU code structure for dcn2x

[Why]
Current FPU code for DCN2x is located under dml/dcn2x.
This is not aligned with DC's general source tree
structure.

[How]
Move FPU code for DCN2x to dml/dcn20.

Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Qingqing Zhuo <qingqing.zhuo@amd.com>