We need to prevent overcommitting the aperture, and in particular if we
allocate a buffer larger than available space we will fail to mmap it in
and rendering will fail. Trying to allocate multiple large buffers in
the aperture, often the case when falling back, causes thrashes and
eviction of useful buffers. So from the outset simply do not allocate a
bo if the the required size is more than half the available aperture
space.

Fixes allocation failure in ocitymap.trace for instance.

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