drm/i915: constify harder

We not only want const strings, but a const array of them. Reported by
checkpatch.pl

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