drm/amd/display: Create dcn321_fpu file

The file dcn321_resource has a lot of FPU operations that should be
inside the dml folder. This commit introduces the dcn321_fpu file and
moves some of the FPU operation functions to this new file.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>