drm/amd/display: Avoid NULL dereference of timing generator

stable inclusion
from stable-v4.19.315
commit 09909f515032fa80b921fd3118efe66b185d10fd
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I9R4M5
CVE: CVE-2023-52753

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=v4.19.315&id=09909f515032fa80b921fd3118efe66b185d10fd



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[ Upstream commit b1904ed480cee3f9f4036ea0e36d139cb5fee2d6 ]

[Why & How]
Check whether assigned timing generator is NULL or not before
accessing its funcs to prevent NULL dereference.

Reviewed-by: Jun Lei <jun.lei@amd.com>
Acked-by: Hersen Wu <hersenxs.wu@amd.com>
Signed-off-by: Wayne Lin <wayne.lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Wenyu Huang <huangwenyu5@huawei.com>