drm/amd/display: move dcn31_update_soc_for_wm_a func to dml fpu folder

Although dcn31_update_soc_for_wm_a() is only called in dml/dcn31/dcn31_fpu by
dc->res_pool->funcs->update_soc_for_wm_a(dc, context), it's declared in
dcn31_resource that is not FPU protected. Move this function to dcn31_fpu
file as part of the work to isolate FPU code.

Signed-off-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>