Instead of hardcoded GL_BGRA8_EXT.
Also change renderTargetFormat to GL_BGRA8_EXT in
DisplayVk::generateConfigs()

BUG=angleproject:2314

Change-Id: I47471add4e681fd0b8101d475c4a33ab59920c5c
Reviewed-on: https://chromium-review.googlesource.com/881952
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>