drm/i915: Add __destroy_hw_context

__create_hw_context can use a good counterpart.

Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1486640065-13695-1-git-send-email-joonas.lahtinen@linux.intel.com