Fix compile error (uninitialized variable use)

switch is already handling all cases, but some compilers don't care.
This suppresses errors about potentially using vkLoadOp and vkStoreOp
uninitialized.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2068523002

Review-Url: https://codereview.chromium.org/2068523002
1 file changed