drm/amd/display: Fix stale buffer object (bo) use

Fixes stale buffer object (bo) usage for cursor plane

Cursor plane's bo operations are handled in DC code.
Currently, atomic_commit() does not handle bo operations
for cursor plane, as a result the bo assigned for cursor
plane in dm_plane_helper_prepare_fb() is not coherent
with the updates to the same made in dc code.This mismatch
leads to "bo" corruption and hence crashes during S3 entry.

This patch cleans up the code which was added as a hack
for 4.9 version only.

Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Pratik Vishwakarma <Pratik.Vishwakarma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>