drm/amd/display: Disable GSL when enabling phantom pipe

[Description]
When enabling phantom pipe on a pipe that was previously
using immediate flip, we have to disable GSL or this will
prevent the update from taking place right away on the phantom
pipe when we enable it in FW

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alvin Lee <Alvin.Lee2@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>