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: stable@vger.kernel.org
Bugzilla: https://bugs.freedesktop.org/106418


Reported-by:  <mikhail.v.gavrilov@gmail.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Junwei Zhang <Jerry.Zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>