The blend factor were not converted correctly from GL enum types to
their VK equivalents. I found this by running many dEQP tests related
to blending.

Bug:angleproject:2346

Change-Id: I3c332bb5e5c44d2bec4ff9d62e0df2421507e136
Reviewed-on: https://chromium-review.googlesource.com/956241
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>