drm/i915: prefer INTEL_GEN() over INTEL_INFO()->gen

Prefer INTEL_GEN() over INTEL_INFO()->gen except in special
circumstances.

v2: don't change device info dump (Chris)

Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180426113521.28417-1-jani.nikula@intel.com