drm/amd/display: Uncomment SubVP pipe split assignment in driver

[Why & How]
Uncomment SubVP pipe split assignment in driver since FW headers
are now promoted

Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Alvin Lee <Alvin.Lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>