drm/rockchip: Use cma gem vm ops

No need to reinvent this little wheel.

v2: Like, try to make it compile even.

Cc: Mark Yao <mark.yao@rock-chips.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464630800-30786-21-git-send-email-daniel.vetter@ffwll.ch