Statically allocate one EGL display.

This reduces support to EGL_DEFAULT_DISPLAY or assumes the
native display handle corresponds with the default display.

Bug 24600445

Change-Id: Ieb5d08beceff1cc46557483e1bed159dd9ab1bee
Reviewed-on: https://swiftshader-review.googlesource.com/4520
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>