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>

drivers/gpu/drm/amd/display/dc/dcn10/dcn10_hw_sequencer.c

@@ -3385,7 +3385,9 @@ static bool dcn10_can_pipe_disable_cursor(struct pipe_ctx *pipe_ctx)
 	for (test_pipe = pipe_ctx->top_pipe; test_pipe;
 	     test_pipe = test_pipe->top_pipe) {
 		// Skip invisible layer and pipe-split plane on same layer
-		if (!test_pipe->plane_state->visible || test_pipe->plane_state->layer_index == cur_layer)
+		if (!test_pipe->plane_state ||
+		    !test_pipe->plane_state->visible ||
+		    test_pipe->plane_state->layer_index == cur_layer)
 			continue;
 
 		r2 = test_pipe->plane_res.scl_data.recout;