drm/radeon: check the alloc_workqueue return value in radeon_crtc_init()

mainline inclusion
from mainline-v6.8-rc1
commit 7a2464fac80d42f6f8819fed97a553e9c2f43310
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I8T5EB
CVE: CVE-2019-16230

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7a2464fac80d42f6f8819fed97a553e9c2f43310



--------------------------------

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>