drm/i915: add constants to size fence arrays and fields

In preparation of to support 32 fences on Ivybdrigde.

Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Keith Packard <keithp@keithp.com>