drm/amd/display: prevent loop from occuring in pipe list

[Why]
If no free pipes are available, acquire_first_split_pipe is called to
get a pipe to use. This call may alter the ordering of the pipes in the
list so that, for example, the tail pipe changes.

If acquire_first_split_pipe returns the tail pipe, we'll have free_pipe
== tail_pipe. What tail_pipe refers to is not the current tail_pipe, but
what was previously the tail pipe - i.e. prior to the call to
acquire_first_split_pipe

The logic that follows will link free_pipe to the tail pipe, referring to
the current tail pipe. However, since tail_pipe is cached from before the
call to acquire_first_split_pipe, the wrong tail pipe will be used, and
it will end up being linked to itself, creating a loop that, if traversed,
will result in a soft hang.

[How]
Do not cache the tail pipe. Instead, check the tail pipe after the call to
acquire_first_split_pipe is made.
Signed-off-by: Josip Pavic <Josip.Pavic@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/core/dc_resource.c

@@ -1358,9 +1358,6 @@ bool dc_add_plane_to_context(
 	dc_plane_state_retain(plane_state);
 
 	while (head_pipe) {
-		tail_pipe = resource_get_tail_pipe(&context->res_ctx, head_pipe);
-		ASSERT(tail_pipe);
-
 		free_pipe = acquire_free_pipe_for_head(context, pool, head_pipe);
 
 	#if defined(CONFIG_DRM_AMD_DC_DCN)
@@ -1378,6 +1375,8 @@ bool dc_add_plane_to_context(
 		free_pipe->plane_state = plane_state;
 
 		if (head_pipe != free_pipe) {
+			tail_pipe = resource_get_tail_pipe(&context->res_ctx, head_pipe);
+			ASSERT(tail_pipe);
 			free_pipe->stream_res.tg = tail_pipe->stream_res.tg;
 			free_pipe->stream_res.abm = tail_pipe->stream_res.abm;
 			free_pipe->stream_res.opp = tail_pipe->stream_res.opp;