intel: enable relaxed fence allocation for i915

The kernel has always allowed userspace to underallocate objects
supplied for fencing. However, the kernel only allocated the object size
for the fence in the GTT and so caused tiling corruption. More recently
the kernel does allocate the full fence region in the GTT for an
under-sized object and so advertises that clients may finally make use
of this feature. The biggest benefit is for texture-heavy GL games on
i945 such as World of Padman which go from needing over 1GiB of RAM to
play to fitting in the GTT!

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