sna: Give kgem_create_buffer_2d a unique id

The gen3, among others, backend uses the unique id of a buffer to track
the currently attached buffer and uses 0 as the invalid value. Linear
buffers as created by kgem_create_buffer_2d were not being assigned a
unique id causing mayhem when they were then being passed to the
backends as render targets and sources. In particular, gen3 did not
notice the switch in render target and did not emit commands to change
the GPU target nor attach the buffer to the batch, causing the
sna_read_boxes to fail and for us to trigger an assertion for an
uncomsumed read buffer.

Reported-by: Clemens Eisserer <linuxhippy@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=42718
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

src/sna/kgem.c

@@ -3178,6 +3178,7 @@ struct kgem_bo *kgem_create_buffer_2d(struct kgem *kgem,
 	}
 
 	bo->pitch = stride;
+	bo->unique_id = kgem_get_unique_id(kgem);
 	return bo;
 }