Vulkan: Fix issue in blending factor conversions 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>