drm/amd/display: Check for flip pending before locking pipes

[Why]
When running games or benchmarking with v-sync disabled, disabling
a plane (which is v-sync) can cause underflow. This is caused by
flips pending before pipe locking being applied after locks are
released and pipes could have been re-arranged or disconnected. This
could potentially apply a flip on incorrect pipe. Also, previous logic
of always locking pipes was unnecessary.

[How]
Only lock the pipes when there is a pipe being disabled to increase
efficiency. Before the pipes are locked, check that any pending flips
are cleared to ensure the flips are applied to the correct pipe.

Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Acked-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>