drm/i915: Rename shrinker init/cleanup to match driver initialisation phase

Since the shrinker is registered and unregistered during
i915_driver_register and i915_driver_unregister, respectively, rename
the init/cleanup functions to match.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171123115338.10270-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>