drm/amd/display: Fix plug/unplug external monitor will hang while playback MPO video

[Why]
Pipes for MPO primary and overlay will be power down and power up during
plug/unplug external monitor while MPO video playback.
But the pipes were the same after plug/unplug and should not need to be
power down and power up or it will make page flip interrupt disabled and
cause hang issue.

[How]
Add pipe split change condition that not only check the top pipe pointer
but also check the index of top pipe if both top pipes are available.

Reviewed-by: Sun peng Li <Sunpeng.Li@amd.com>
Acked-by: Brian Chang <Brian.Chang@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>