drm/amd/display: Add wait for flip not pending on pipe unlock

[Why]
Lack of proper timing caused intermittent underflow on unplug external DP.
A previous fix was invalid and caused S0i3 regression, so had to be reverted.

[How]
When unlocking pipe, wait for no pipes to have flip pending before unlocking.

Signed-off-by: Noah Abradjian <noah.abradjian@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>