drm/amd/display: fix dereferencing possible ERR_PTR()

This patch fixes static checker warning caused by
"36cc549d: "drm/amd/display: disable CRTCs with
NULL FB on their primary plane (V2)"

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Shirish S <shirish.s@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>