drm/i915/cnl: add IS_CNL_REVID macro

We're going to use it in the next commits.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Jim Bride <jim.bride@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1496781040-20888-5-git-send-email-rodrigo.vivi@intel.com