drm/i915: Promote IS_BROADWELL to a simple macro

If we allow it a dedicated flag in dev_priv we enable the
compiler to nicely optimize conditions like IS_HASSWELL ||
IS_BROADWELL.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>