drm/amd/display: Move bw_fixed from DML folder

bw_fixed does not need any FPU operation, and it is used on DCE and DCN.
For this reason, this commit moves bw_fixed to the basic folder outside
DML.

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>