drm/amd/display: Fix dml2 assigned pipe search

[Why & How]
DML2 currently finds assigned pipes in array order rather than the
existing linked list order. This results in rearranging pipe order
on flip and more importantly otg inst and pipe idx mismatch.

This change preserves the order of existing pipes and guarantees
the head pipe will have matching otg inst and pipe idx.

Reviewed-by: Gabe Teeger <gabe.teeger@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Dmytro Laktyushkin <dmytro.laktyushkin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>