drm/i915: Add IS_BDW_GT3 macro.

It will be usefull to specify w/a that affects only BDW GT3.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>