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>