Try adding the playlist parameter along with the loop. For playlist, set it’s value as the current video id.
<iframe class="embed-responsive-item"id="ytplayer" type="text/html" width="640" height="360" src="https://www.youtube.com/embed/M7lc1UVf-VE?&autoplay=1&loop=1&rel=0&showinfo=0&color=white&iv_load_policy=3&playlist=M7lc1UVf-VE"
frameborder="0" allowfullscreen></iframe>
Currently, the loop parameter only works in the AS3 player when used in conjunction with the playlist parameter. To loop a single video, set the loop parameter value to 1 and set the playlist parameter value to the same video ID already specified in the Player API URL:
http://www.youtube.com/v/VIDEO_ID?version=3&loop=1&playlist=VIDEO_ID
Reference:https://developers.google.com/youtube/player_parameters#loop