draw: cleanup and fix instance id computation

draw: cleanup and fix instance id computation

The instance id system value always starts at 0, even if the
specified start instance is larger than 0. Instead of implicitly
setting instance id to instance id plus start instance and then
having to subtract instance id when computing the buffer offsets
lets just set instance id to the proper instance id. This fixes
instance id computation and cleansup buffer offset computation.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>