drm/i915: Replace some negative Gen checks

Instead of INTEL_GEN != x use !IS_GENx for more optimisation
opportunities.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180208130606.15556-16-tvrtko.ursulin@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180209215847.6660-2-chris@chris-wilson.co.uk