drm/armada: use unlocked gem unreferencing

For drm_gem_object_unreference callers are required to hold
dev->struct_mutex, which these paths don't. Enforcing this requirement
has become a bit more strict with

commit ef4c6270
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Oct 15 09:36:25 2015 +0200

    drm/gem: Check locking in drm_gem_object_unreference

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>