drm/i915/bdw: add IS_BROADWELL macro

For now it's just equivalent to IS_GEN8, but in the future we might
want to change that (e.g., on Gen 7 we have IS_VALLEYVIEW,
IS_IVYBRIDGE and IS_HASWELL).

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>