svga: avoid provoking vertex conversion when possible

svga: avoid provoking vertex conversion when possible

Provoking vertex comes into play when doing flat shading.  But if we know
that all fragments in a primitive are the same color, the provoking vertex
doesn't matter.  Check for that case and use whichever provoking vertex
convention is supported by the device.

This avoids generating an index buffer to do the PV conversion.

Reviewed-by: Charmaine Lee <charmainel@vmware.com>
Reviewed-by: José Fonseca <jfonseca@vmware.com>