drm/amd/display: fix incorrect stream_res allocation for older ASIC

[why]
There is a recent work for developing a new pipe resource allocation
policy used for new ASIC. The new code change needs to modify asic
independent pipe resource allocation flow and hook up the new allocation
policy in asic dependent layer. Unfortunately this change revealed a
hidden bug in the old pipe resource allocation sequence used for older
asics. In the older version of acquiring pipe for layer, we are always
assigning otg master's opp and tg to the newly allocated secondary dpp
pipe. This logic is incorrect when the secodnary dpp
pipe is connected to a secondary opp head pipe in ODM combine
configuration. Before the recent change, we will overwrite this wrong
assignement in asic independent layer again. This covers up the issue.
With the recent change, we will no longer cover up this in upper layer
and therefore causes wrong tg and opp assignement to the secondary
dpp pipe connected to a secondary opp head.

[how]
Always assign tg and opp from its own opp head instead of otg master.

Reviewed-by: Martin Leung <martin.leung@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>