drm/amd/display: Move specific DCN2x code that uses FPU to DML

The display core files rely on FPU, which requires to be compiled with
special flags. Ideally, we don't want these FPU operations spread around
the DC code; nevertheless, it happens in the current source. This commit
introduces a new directory inside DML for centralizing shared DCN
functions that require FPU and have been used outside DML. For
illustrating this process of transferring FPU functions to the DML
folder, this commit moves one of the functions
dcn20_populate_dml_writeback_from_context) that require FPU access to a
single shared file. Notice that this is the first part of the work, and
it does not fix the FPU issue yet; we still need other patches for
achieving the complete FPU isolation.

Changes since V3:
- Jun: Instead of creating a new directory to keep the FPU code, let's
make the DML folder the only part that requires FPU access. Drop
fpu_operation folder.
- Christian: Fix function code style.

Changes since V2:
- Christian: Remove unnecessary wrapper.
- lkp: Add missing prototype.
- Only compile the FPU operations if the DCN option is enabled.

Change since V1:
- Update documentation and rebase.

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Anson Jacob <Anson.Jacob@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Hersen Wu <hersenxs.wu@amd.com>
Cc: Aric Cyr <aric.cyr@amd.com>
Cc: Jun Lei <jun.lei@amd.com>
Cc: Dmytro Laktyushkin <dmytro.laktyushkin@amd.com>
Cc: Qingqing Zhuo <qingqing.zhuo@amd.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>