drm/i915: Tidy gen6_write_pde()

Stop passing around unused parameters makes the code more compact.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170215084357.19977-11-chris@chris-wilson.co.uk