drm/i915: Add IS_SKL_GT3 and IS_SKL_GT4 macro.

It will be usefull to specify w/a that affects only SKL GT3 and GT4.

Signed-off-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Reviewed-by: Alex Dai <yu.dai@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>