drm/i915: Drop struct_mutex around frontbuffer flushes

Since the frontbuffer has self-contained locking, it does not require us
to hold the BKL struct_mutex as we send invalidate and flush messages.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170215105919.7347-3-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>