Fixes warnings about switch statements not handling all enum values.

Bug: b/123933266
Change-Id: I0372ebf2412452622eb99ea7bba7ebc5cedd385c
Reviewed-on: https://swiftshader-review.googlesource.com/c/25009Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>