drm/i915: Markup GEM API with lockdep asserts

Add lockdep_assert_held(struct_mutex) to the API preamble of the
internal GEM interfaces.

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/20161028125858.23563-9-chris@chris-wilson.co.uk