drm/amdgpu: fix preamble handling

drm/amdgpu: fix preamble handling

BugLink: https://bugs.launchpad.net/bugs/1836287
[ Upstream commit d98ff24e8e9be3329eea7c84d5e244d0c1cd0ab3 ]

At this point the command submission can still be interrupted.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>