drm/i915: tidy up gen8_init_scratch

Prefer a goto teardown path to do all the required cleanup.

v2:
(Joonas Lahtinen)
  - remove NULL assignments
  - rename goto labels

Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Dave Gordon <david.s.gordon@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461759565-12086-1-git-send-email-matthew.auld@intel.com