drm/omap: gem: Switch to gem_free_object_unlocked()

The only thing that omap_gem_free_object does that might need the magic
protection of struct_mutex (of keeping all objects alive if that lock is
held, even if the last reference is gone) is the mm_list manipulation.
This is already protected by the separate omapdrm->list_lock, which
means that struct_mutex is not needed by omapdrm. We can switch to
gem_free_object_unlocked()

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>