drm/i915: Not all mappable regions require GTT fence regions

Combining map_and_fenceable revealed a bug in
i915_gem_object_gtt_size() in that it always computed the appropriate
fence size for the object regardless of tiling state which caused us to
over-allocate linear buffers when binding to the GTT.

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