drm/amd/display: make virtual_disable_link_output static

It's not used outside of virtual_link_hwss.c. Fixes
a -Wmissing-prototypes warning.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drm/amd/display: rework recent update PHY state commit

[why]
Original change 594b237b9a07 ("drm/amd/display: Add
interface to track PHY state") was implemented by assuming stream's
dpms off is equivalent to PHY power off.
This assumption doesn't hold in following situations:
1. MST multiple stream scenario, where multiple streams are sharing the
same PHY output. Toggle dpms off for one of the stream doesn't power
off the PHY due to the presence of other streams.
2. enable stream failure scenario, where enable stream fails due to
failure of link training. This will cause DPMS off is set to false, while
the actual PHY power state is off in certain cases.
Due to the problematic assumption, the logic will skip disabling
other streams for MST multiple stream scenario, therefore PHY is
not actually powered off.

[how]
1. Rework this refactor by moving PHY state update down to hardware
level, where we update PHY state in place when hardware sequencer
is actually changing the power state of the PHY hardware.
2. Reimplement symclk on TX off workaround in place when we are actually
calling transmitter control to power off PHY in dcn32. Note the workaround is
added due to the lack of proper software interface to set TX while keeping
symclk on. We plan to address this interface problem so we can set TX off
only without affecting symclk in future dcn versions.

Fixes: 594b237b9a07 ("drm/amd/display: Add interface to track PHY state")
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drm/amd/display: extract set stream attribute to link_hwss

[why]
Extract set stream attribute into link_hwss as part of the link hwss
refactor work.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: George Shen <George.Shen@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drm/amd/display: move link_hwss to link folder and break down to files

[why]
Move link_hwss to its own folder as part of DC LIB and break it down
to separate file one for each type of backend for code isolation.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>