drm/i915/dp: Don't switch the LTTPR mode on an active link

[ Upstream commit 509580fad7323b6a5da27e8365cd488f3b57210e ]

Switching to transparent mode leads to a loss of link synchronization,
so prevent doing this on an active link. This happened at least on an
Intel N100 system / DELL UD22 dock, the LTTPR residing either on the
host or the dock. To fix the issue, keep the current mode on an active
link, adjusting the LTTPR count accordingly (resetting it to 0 in
transparent mode).

v2: Adjust code comment during link training about reiniting the LTTPRs.
   (Ville)

Fixes: 7b2a4ab8

 ("drm/i915: Switch to LTTPR transparent mode link training")
Reported-and-tested-by: Gareth Yu <gareth.yu@intel.com>
Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/10902
Cc: <stable@vger.kernel.org> # v5.15+
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240708190029.271247-3-imre.deak@intel.com
(cherry picked from commit 211ad49cf8ccfdc798a719b4d1e000d0a8a9e588)
Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>