drm/exynos: Convert from atomic_t to refcount_t on g2d_cmdlist_userptr->refcount

refcount_t type and corresponding API can protect refcounters from
accidental underflow and overflow and further use-after-free situations.

Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>