drm/i915: Remove more ilk rc6 remnants

Leftover from the big purge

commit a5611654
Author: John Harrison <John.C.Harrison@Intel.com>
Date:   Thu Mar 5 14:03:03 2015 +0000

    drm/i915: Remove ironlake rc6 support

Cc: John Harrison <John.C.Harrison@Intel.com>
Cc: Rob Clark <robdclark@gmail.com>
Reported-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>