drm/amd/display: clear mpc_tree in init_pipes

drm/amd/display: clear mpc_tree in init_pipes

[Why]
During init_pipes, otg master is not initialized. So mpc tree is
still configured even if mpc bottom is not active

[How]
For pipes that have tg enabled, check their mpc tree and clear
opp_list if mpc bottom is not active

Reviewed-by: George Shen <george.shen@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Samson Tam <samson.tam@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>