drm/radeon: check the alloc_workqueue return value

hulk inclusion
category: bugfix
bugzilla: 13690
CVE: CVE-2019-16230

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check the alloc_workqueue return value in radeon_crtc_init()
to avoid null-ptr-deref.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Hanjun Guo <guohanjun@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>