Refactor vulkan buffer mapping and unmapping

A lot of this is so we don't have duplicated code in both the map/unmap and
updateData functions of GrVkBuffer. Also there were slightly differences in
how we handled various things in the two cases that this now unifies.

Also I added a barrier after the vkUpdateBuffer call which I believe was
missing.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2344323002

Review-Url: https://codereview.chromium.org/2344323002
4 files changed