intel: Compute in-aperture size for relaxed fenced objects

For relaxed fencing the object may only consume the small set of active
pages, but still requires a fence region once bound into the aperture.
This is the size we need to use when computing the maximum possible
aperture space that could be used by a single batchbuffer and so avoid
hitting ENOSPC.

Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>