drm/amdgpu: handle gang submit before VMID

Otherwise it can happen that not all gang members can get a VMID
assigned and we deadlock.

Signed-off-by: Christian König <christian.koenig@amd.com>
Tested-by: Timur Kristóf <timur.kristof@gmail.com>
Acked-by: Timur Kristóf <timur.kristof@gmail.com>
Fixes: 68ce8b24

 ("drm/amdgpu: add gang submit backend v2")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221118153023.312582-1-christian.koenig@amd.com