nvc0: fix !index_bounds_valid path with user vbufs

Unreal Tournament (1999) hit the else branch of nvc0_user_vbuf_range
hitting the "nvc0->vb_elt_limit != ~0" assert.

Setting proper values for vb_elt_first and vb_elt_limit fixes this.

Closes: #7028
Cc: mesa-stable
Signed-off-by: Karol Herbst <kherbst@redhat.com>
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