virgl: define MAX_VERTEX_STREAMS based on availability of TF3

virgl: define MAX_VERTEX_STREAMS based on availability of TF3

Since with gles hosts we lie about the GLSL feature level it is better
to set the number of streams based on actual hosts capabilities.

v2: Make use of feature check level to avoid regressions.

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-By:  Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>