drm/i915: Replace hardcoded cacheline size with macro

For readibility and guess at the meaning behind the constants.

v2: Claim only the meagerest connections with reality.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Oscar Mateo <oscar.mateo@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>