drm/amd/display: Move custom_float from DML folder

The custom_float file does not have any FPU operation, so it should be
inside DML. This commit moves the file to the basic folder.

Reviewed-by: Qingqing Zhuo <Qingqing.Zhuo@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>