drm/radeon: fix a possible null pointer dereference

mainline inclusion
from mainline-v5.19-rc1
commit a2b28708
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I9R4KL
CVE: CVE-2022-48710

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a2b28708b645c5632dc93669ab06e97874c8244f



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In radeon_fp_native_mode(), the return value of drm_mode_duplicate()
is assigned to mode, which will lead to a NULL pointer dereference
on failure of drm_mode_duplicate(). Add a check to avoid npd.

The failure status of drm_cvt_mode() on the other path is checked too.

Signed-off-by: Gong Yuanjun <ruc_gongyuanjun@163.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Liu Chuang <liuchuang40@huawei.com>