drm/i915: Introduce INTEL_GEN_MASK

Split INTEL_GEN_MASK out of IS_GEN macro, and make it usable
within static declarations (unlike compound statements).

v2:
- s/combound/compound/ (Tvrtko)
- Fix whitespace (yes, we need automatic checkpatch.pl)

Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170913115255.13851-1-joonas.lahtinen@linux.intel.com