drm/amd/display: Add NULL plane_state check for cursor disable logic

[Why]
While scanning the top_pipe connections we can run into a case where
the bottom pipe is still connected to a top_pipe but with a NULL
plane_state.

[How]
Treat a NULL plane_state the same as the plane being invisible for
pipe cursor disable logic.

Cc: stable@vger.kernel.org
Cc: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>