gallium: Fix the ->set_damage_region() implementation

BACK_LEFT attachment can be outdated when the user calls
KHR_partial_update() (->lastStamp != ->texture_stamp), leading to a
damage region update on the wrong pipe_resource object.
Let's delay the ->set_damage_region() call until the attachments are
updated when we're in that case.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>