drm/amdgpu: use the last IB as gang leader v2

It turned out that not the last IB specified is the gang leader,
but instead the last job allocated.

This is a bit unfortunate and not very intuitive for the CS
interface, so try to fix this.

Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221115094206.6181-1-christian.koenig@amd.com
Tested-by: Timur Kristóf <timur.kristof@gmail.com>
Acked-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Fixes: 4624459c ("drm/amdgpu: add gang submit frontend v6")