drm/amd/display: Do not disable phantom pipes in driver

[Why and How]
We should not disable phantom pipes in this sequence, as this should be
controlled by FW. Furthermore, the previous programming sequence would
have enabled the phantom pipe in driver as well, causing corruption.
This change should avoid this from occuring.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Acked-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Saaem Rizvi <syedsaaem.rizvi@amd.com>
Reviewed-by: Alvin Lee <Alvin.Lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>