drm/bridge: tc358767: use more reliable seq when finishing LT

At the end of the link training, two steps have to be taken: 1)
tc358767's LT mode is disabled by a write to DP0_SRCCTRL, and 2) Remove
LT flag in DPCD 0x102.

Toshiba's documentation tells to first write the DPCD, then modify
DP0_SRCCTRL. In my testing this often causes issues, and the link
disconnects right after those steps.

If I reverse the steps, it works every time. There's a chance that this
is DP sink specific, though, but as my testing shows this sequence to be
much more reliable, let's change it.

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/20190528082747.3631-15-tomi.valkeinen@ti.com