gpu: drm: bridge: analogix: analogix_dp_reg: Remove unused function 'analogix_dp_writ...

gpu: drm: bridge: analogix: analogix_dp_reg: Remove unused function 'analogix_dp_write_byte_to_dpcd'

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/bridge/analogix/analogix_dp_reg.c:571:5: warning: no previous prototype for ‘analogix_dp_write_byte_to_dpcd’ [-Wmissing-prototypes]

Cc: Andrzej Hajda <a.hajda@samsung.com>
Cc: Neil Armstrong <narmstrong@baylibre.com>
Cc: Laurent Pinchart <Laurent.pinchart@ideasonboard.com>
Cc: Jonas Karlman <jonas@kwiboo.se>
Cc: Jernej Skrabec <jernej.skrabec@siol.net>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Jason Yan <yanaijie@huawei.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Jingoo Han <jg1.han@samsung.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201105144517.1826692-13-lee.jones@linaro.org