Fix retrieving the current Display.

Fix retrieving the current Display.

eglGetCurrentDisplay() returned a pointer to the concrete egl::Display
object, instead of the opaque identifier obtained from eglGetDisplay.
This was a regression caused by
https://swiftshader-review.googlesource.com/10188

Bug chromium:738298

Change-Id: Id3a87fc3978f8f4efdc77d6c5eaa85743fa3672c
Reviewed-on: https://swiftshader-review.googlesource.com/10508
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>