drm/i915/chv: Add IS_CHERRYVIEW() macro

We will treat Cherryview like Valleyview for most parts. Add a macro
for cases when we need to tell the two apart.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>