drm/i915: Fix the confusing comment about the ioctl limits

It was reported that this comment was confusing, and indeed it is.

v2: (one year later!) Add the range for the DRM_I915_* iotcl defines
    (Daniel)

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>