drm/i915: Remove __I915__ magic macro

And at the same time introduce a static inline helper for
more type safety.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1479286545-15020-2-git-send-email-tvrtko.ursulin@linux.intel.com