drm/i915/skl: Add an IS_SKYLAKE macro

Adding new macro IS_SKYLAKE for skylake specific implementation.

Reviewed-by: Thomas Wood <thomas.wood@intel.com>
Signed-off-by: Satheeshakrishna M <satheeshakrishna.m@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>