[ios] Force update from display link when display link is created.

This forces the call of the map view's display link target when the
display link is created. When a map view is shown the first time
this is not required. However, if the display link was torn down
and then recreated due to the map view being removed / obscured in the
view hierarchy the map view delegate may expect to be informed when
the map view did finish loading the map when it is shown again.

This adds a call to `updateFromDisplayLink` just after a display link
is created which helps trigger a full refresh of the map in the sense
that all delegate methods get exercised including
`mapViewDidFinishLoadingMap`.

platform/ios/src/MGLMapView.mm

@@ -913,6 +913,7 @@ mbgl::Duration MGLDurationInSeconds(NSTimeInterval duration)
         _displayLink.frameInterval = MGLTargetFrameInterval;
         [_displayLink addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSRunLoopCommonModes];
         _needsDisplayRefresh = YES;
+        [self updateFromDisplayLink];
     }
     else if ( ! isVisible && _displayLink)
     {