svga: try to avoid index generation for some primitive types

svga: try to avoid index generation for some primitive types

The svga device doesn't directly support quads, quad strips or polygons
so we have to convert those types to indexed triangle lists.  But we
can sometimes avoid that if we're drawing flat/constant-colored prims
and we don't have to worry about provoking vertex.

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