drm/dp_helper: don't return EPROTO for defers (v2)

If we get a msg.reply of REPLY_DEFER, we also get an err of 0
so we fail reads with 0 < size and return -EPROTO instead of trying
again.

v2: same fix in i2c code.

Found writing MST support.

Reviewed-by: Jani Nikula <[e-mail]>
Reviewed-by: Thierry Reding <[e-mail]>
Signed-off-by: Dave Airlie <[e-mail]>

drivers/gpu/drm/drm_dp_helper.c

···
 			return err;
 		}
 
-		if (err < size)
-			return -EPROTO;
 
 		switch (msg.reply & DP_AUX_NATIVE_REPLY_MASK) {
 		case DP_AUX_NATIVE_REPLY_ACK:
+			if (err < size)
+				return -EPROTO;
 			return err;
 
 		case DP_AUX_NATIVE_REPLY_NACK:
···
 			return err;
 		}
 
-		if (err < msg->size)
-			return -EPROTO;
 
 		switch (msg->reply & DP_AUX_NATIVE_REPLY_MASK) {
 		case DP_AUX_NATIVE_REPLY_ACK:
···
 			 * Both native ACK and I2C ACK replies received. We
 			 * can assume the transfer was successful.
 			 */
+			if (err < msg->size)
+				return -EPROTO;
 			return 0;
 
 		case DP_AUX_I2C_REPLY_NACK: