drm/amd/display: Free atomic state after drm_atomic_commit

[why]
drm_atomic_commit was changed so that the caller must free their
drm_atomic_state reference on successes.

[how]
Add drm_atomic_commit_put after drm_atomic_commit call in
dm_force_atomic_commit.

Signed-off-by: Victor Lu <victorchengchi.lu@amd.com>
Reviewed-by: Roman Li <Roman.Li@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>