drm/amd/display: revert wait in pipelock

[why]
Previous workaround to prevent a vsync flip to be converted
to immediate flip is no longer needed, and is risky because
there are cases where it can result in infinite loop.

[how]
Remove wait loop (which is potentially infinite) before locking
pipe

Signed-off-by: Jun Lei <Jun.Lei@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>