drm/amd/display: Set flip pending for pipe split

[WHY]
When doing split pipe, if one pipe is pending on flip, the entire
plane's status should be flip pending, otherwise corruption can occur
when OS writes to a surface prematurely.

[HOW]
Clear the flip pending bit before checking pipes, then OR the flip
pending bits from all pipes together to create the flip pending status
of the entire plane.

Signed-off-by: Wesley Chalmers <Wesley.Chalmers@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Eryk Brol <Eryk.Brol@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>