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

Fixes: fa7f517c ("drm/radeon: rework page flip handling v4")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>