drm/amd/display: fix nullptr on vega initialization

Signed-off-by: Jordan Lazare <Jordan.Lazare@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Reviewed-by: Roman Li <Roman.Li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>