drm/i915: Don't declare unnecessary shadowing variable

'i' is already defined in the function scope and used elsewhere. Let's
use it instead.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>