[why]
dc_stream_state containing a pointer to sink is poor design.
Sink describes the display, and the specifications or capabilities
it has.  That information is irrelevant for dc_stream_state, which describes
hardware state, and is generally used for hardware programming.  It
could further be argued that dc_sink itself is just a convenience dc
provides, and DC should be perfectly capable of programming hardware
without any dc_sinks (for example, emulated sinks).

[how]
Phase 1:
Deprecate use of dc_sink pointer in dc_stream.  Most references are trivial
to remove, but some call sites are risky (such as is_timing_changed) with
no obvious logical replacement.  These will be removed in follow up change.

Add dc_link pointer to dc_stream.  This is the typical reason DC really needed
sink pointer, and most call sites are replaced with this.

DMs also need minor updates, as all 3 DMs leverage stream->sink for
some functionality.  this is replaced instead by a pointer to private data
inside dc_stream_state, which is used by DMs as a quality of life improvment
for some key functionality.  it allows DMs to set pointers have to their own objects
which associate OS objects to dc_stream_states (such as DisplayTarget
and amdgpu_dm_connector).  Without the private pointer, DMs would be
forced to perform a lookup for callbacks.
Signed-off-by: Jun Lei <Jun.Lei@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: David Francis <David.Francis@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>