drm/amd/display: use correct vpg instance for 128b/132b encoding

[why]
128b/132b uses the vpg instance assigned to hpo dp stream encoder.
The current vpg used is assigned to dio stream encoder.
This is incorrect and cause display black screen because the
actual vpg is powered off.

Reviewed-by: Michael Strauss <michael.strauss@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@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>