drm/amdgpu: Reserve VM root shared fence slot for command submission (v3)

drm/amdgpu: Reserve VM root shared fence slot for command submission (v3)

Without this, there could not be enough slots, which could trigger the
BUG_ON in reservation_object_add_shared_fence.

v2:
* Jump to the error label instead of returning directly (Jerry Zhang)
v3:
* Reserve slots for command submission after VM updates (Christian König)

Cc: [email protected]
Bugzilla: https://bugs.freedesktop.org/106418
Reported-by: [email protected]
Signed-off-by: Michel Dänzer <[email protected]>
Signed-off-by: Junwei Zhang <[email protected]>
Reviewed-by: Christian König <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>