drm/amd/display: System Hang When System enters to S0i3 w/ iGPU

drm/amd/display: System Hang When System enters to S0i3 w/ iGPU

[why]
System Hang when system enters to S0i3 w/ iGPU
some link_enc are NULL due to BIOS integration info table not correct,
but driver should have enough null pointer protection.

Reviewed-by: Leo Chen <leo.chen@amd.com>
Signed-off-by: Charlene Liu <Charlene.Liu@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Signed-off-by: Chuanyu Tseng <Chuanyu.Tseng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>