drm/amd/display: Skip finding free audio for unknown engine_id

stable inclusion
from stable-v4.19.318
commit 9eb4db08a808e3a3ba59193aeb84a57a6dc4d8c9
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/IAGT0E
CVE: CVE-2024-42119

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9eb4db08a808e3a3ba59193aeb84a57a6dc4d8c9



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[ Upstream commit 1357b2165d9ad94faa4c4a20d5e2ce29c2ff29c3 ]

[WHY]
ENGINE_ID_UNKNOWN = -1 and can not be used as an array index. Plus, it
also means it is uninitialized and does not need free audio.

[HOW]
Skip and return NULL.

This fixes 2 OVERRUN issues reported by Coverity.

Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Zhao Wenhui <zhaowenhui8@huawei.com>