drm/amd/display: wait for pending complete when enabling a plane

[why]
When planes are enabled, they must be enabled using VSYNC update (not immediate).
However, before the VUPDATE occurs, DM may call with an "immediate" flip which is address
only.  This operation would normally be okay, but if the locking for immediate flip happens
to occur before the VUPDATE associated with the initial plane enablement, it will cause HW
to hang.

[how]
HWSS should enforce plane enable in HW to be synchronous with the call that enables the plane.

Signed-off-by: Jun Lei <Jun.Lei@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>