drm/i915: Don't cast a pointer to void* unnecessarily

C is super happy to asign anything pointer to void *. Don't pretend
otherwise.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>