EGL functions are supposed to return EGL_BAD_DISPLAY when the display is
an invalid pointer.

Since there is very rarely more than one simultaneous display, iterating
through the map is not costly.

BUG=angleproject:970

Change-Id: I3d9b842a49c5c99f9b8fad3d7ef53f9f5d3057ba
Reviewed-on: https://chromium-review.googlesource.com/287342Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>