drm/bridge: tc358767: fix ansi 8b10b use

DP always uses ANSI 8B10B encoding. Some monitors (old?) may not have
the ANSI 8B10B bit set in DPCD, even if it should always be set.

The tc358767 driver currently respects that flag, and turns the encoding
off if the monitor does not have the bit set, which then results in the
monitor not working.

This patch makes the driver to always use ANSI 8B10B encoding, and drops
the 'coding8b10b' field which is no longer used.

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/20190528082747.3631-4-tomi.valkeinen@ti.com