drm/amd/display: fix pipe topology logging error

[why]
There is a logging error in the recently added pipe topology log.
If the plane with index 0 uses MPC combine, the log shows that
as two separate planes.

[how]
Initialize plane idx as -1 and increment plane idx before logging
any primary dpp pipes of a plane.

Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>