drm/bridge: Prepare Analogix anx6345 support

Add bit definitions required for the anx6345 and add a
sanity check in anx_dp_aux_transfer.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Signed-off-by: Torsten Duwe <duwe@suse.de>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107135218.01C2168C4E@verein.lst.de

drivers/gpu/drm/bridge/analogix/analogix-i2c-dptx.c

@@ -116,7 +116,7 @@ ssize_t anx_dp_aux_transfer(struct regmap *map_dptx,
 	else	/* For non-zero-sized set the length field. */
 		ctrl1 |= (msg->size - 1) << SP_AUX_LENGTH_SHIFT;
 
-	if ((msg->request & DP_AUX_I2C_READ) == 0) {
+	if ((msg->size > 0) && ((msg->request & DP_AUX_I2C_READ) == 0)) {
 		/* When WRITE | MOT write values to data buffer */
 		err = regmap_bulk_write(map_dptx,
 					SP_DP_BUF_DATA0_REG, buffer,