drm/armada: use a private mutex to protect priv->linear

Reusing the Big DRM Lock just leaks, and the few things left that
dev->struct_mutex protected are very well contained - it's just the
linear drm_mm manager.

With this armada is completely struct_mutex free!

v2: Convert things properly and also take the lock in
armada_gem_free_object, and remove the stale comment (Russell).

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