drm/amd/display: Missing HPO instance added

[Why & How]
Number of encoder is set to 4 but only 3 instances are created.

Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Brian Chang <Brian.Chang@amd.com>
Signed-off-by: Leo Chen <sancchen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>