drm/rockchip: gem: add the lacks lock and trivial changes

As the allocation and free buffer that need to add mutex lock for drm mm,
but it lacks the locking on error path in rockchip_gem_iommu_map().
Also, the trivial changes like The comment should be  placed in the
kerneldoc and unused blank line.

Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Reviewed-by: Mark Yao <mark.yao@rock-chips.com>
Signed-off-by: Mark Yao <mark.yao@rock-chips.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1496196863-25738-1-git-send-email-wxt@rock-chips.com