D3D11: Fix loading of RGB5A1 data from RGBA8. BUG=angleproject:1407 BUG=chromium:616176 Change-Id: I2b88616b6b72dd9caf07fac8ebba9a065cf2983f Reviewed-on: https://chromium-review.googlesource.com/350907 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>