Disable the use of this extension on Mesa 20/intel, it's causing GPU
hangs.

Bug: chromium:1187475
Change-Id: Ic7029b1f72bd0b0f4a2c11a7fceb0a59f51be8aa
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2920811Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>