drm/i915: simplify sysfs setup code

Positively checking for the required feature/gen is simpler than build
a cascade of negative "we need to bail" checks. And the later won't
scale if we add more stuff that doesn't fit in nicely.

Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>