r600g: fix rendering with a vertex attrib having a zero stride
r600g: fix rendering with a vertex attrib having a zero stride

The hardware supports zero stride just fine.  This is a port
of 2af8a1983180fc0168c1e0e53bcc69ee3d684ea4 from r300g.

NOTE: This is a candidate for both the 7.9 and 7.10 branches.

Signed-off-by: Alex Deucher <bf84f557a1e54fd62cd73c6847f7d9df60e9fbef@gmail.com>

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