vulkan/wsi: Do image creation in common code

vulkan/wsi: Do image creation in common code

This uses the mock extension created in a previous commit to tell the
driver that the image it's just been asked to create is, in fact, a
window system image with whatever assumptions that implies.  There was a
lot of redundant code between the two drivers to do basically exactly
the same thing.

Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Chad Versace <chadversary@chromium.org>