Cast the second parameter of eglGetPlatformDisplayEXT() to the correct
type to stop compiler complaining.

BUG=angleproject:1297

Change-Id: I37260edbd0fb6b92c24b556b15ea8206295e12d4
Reviewed-on: https://chromium-review.googlesource.com/354740Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>