drm/amd/display: add NULL pointer check

[why]
check dmub_Srv exist or not before accessing dmub.

Acked-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Charlene Liu <charlene.liu@amd.com>
Reviewed-by: Zhan Liu <Zhan.Liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>