drm/i915/gtt: Cleanup page directory encoding

Write page directory entry without using superfluous
indirect function. Also remove unused device parameter
from the encode function.

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>