drm/amd/display: Add NULL-checks in dml2 assigned pipe search

[Why]
NULL-deref regression after:
"drm/amd/display: Fix dml2 assigned pipe search"

[How]
Add verification for potential NULLs

Fixes: d451b534 ("drm/amd/display: Fix dml2 assigned pipe search")
Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Roman Li <roman.li@amd.com>
Signed-off-by: Gabe Teeger <gabe.teeger@amd.com>
Signed-off-by: Allen Pan <allen.pan@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>