drm/bridge: tc358767: ensure DP is disabled before LT

Link training will sometimes fail if the DP link is enabled when
tc_main_link_enable() is called. The driver makes sure the DP link is
disabled when the DP output is disabled, and we never enable the DP
without first disabling it, so this should never happen.

However, as the HW behavior seems to be somewhat random if DP link has
erroneously been left enabled, let's add a WARN_ON() for the case and
set DP0CTL to 0.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190528082747.3631-13-tomi.valkeinen@ti.com