drm/amd/display: fix not calling ppsmu to trigger PME

[why]
dcn20_clk_mgr_construct was not initializing pp_smu, and PME call gets
filtered out by the null check

[how]
initialize pp_smu dcn20_clk_mgr_construct

Signed-off-by: Su Sung Chung <Su.Chung@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>