Don't silently error if damage is sent before buffer

Summary:
Firefox sends

wl_surface@37.damage(0, 0, 808, 622)
wl_surface@37.attach(wl_buffer@34, 0, 0)

Which we silently treat as an error.

There's nothing in the spec to forbid this. The only thing that matters
is the state on commit. This moves a check there.

CCBUG: 397834

Test Plan:
Had a debug in there which was being activated
Gets firefox slightly further (but not complete)

Reviewers: #kwin

Subscribers: kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D15912