sna/dri2: Recreate buffer in reuse rather than force recreate

sna/dri2: Recreate buffer in reuse rather than force recreate

By marking the buffer->attachment as invalid we forced the recreation of
the buffer on the next DRI2GetBuffers. The only small problem with that
was that the client didn't always immediately call DRI2GetBuffers after
the invalidation - and if they did a DRI2CopyRegion with its stale
buffers the unfound attachment would generate a BadValue and kill the
client. So instead of messing around with the attachment, explicitly
recreate the buffer if we attempt to reuse a buffer of the wrong size.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>