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

[Why]
When running a game/benchmark with v-sync disabled, disabling a plane
(which is v-sync) can cause an underflow. This is due to flips that are
pending before pipe locking being applied after locks are released and
pipes have been re-arranged or disconnected. This can potentially apply
a flip on the incorrect pipe.

[How]
Check that any pending flips are cleared before locking any pipes to
ensure flips are applied on the correct pipes.

Signed-off-by: Taimur Hassan <syed.hassan@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>