drm/i915: Introduce IS_GEN macro

To be used for more efficient Gen range checking.

v2: Remove spurious chunk. (Chris Wilson)
v3: Rebase.
v4: Renamed from INTEL_GEN_RANGE and added GEN_FOREVER.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> (v3)
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Dave Gordon <david.s.gordon@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1462874228-6601-1-git-send-email-tvrtko.ursulin@linux.intel.com