drm/i915: Remove unused ppgtt->enable()

We never assign or use the ppgtt->enable() callback, so remove it.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170215084357.19977-19-chris@chris-wilson.co.uk