drm/armada: drop struct_mutex from cursor paths

The kms state itself is already protected by the modeset locks
acquired by the drm core. The only thing left is gem bo state, and
since the cursor code expects small objects which are statically
mapped at create time and then invariant over the lifetime of the gem
bo there's nothing to protect.

See armada_gem_dumb_create -> armada_gem_linear_back which assigns
obj->addr which is the only thing used by the cursor code.

Only tricky bit is to switch to the _unlocked unreference function.

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