nv50: improve broken vbo handling
What does this MR do and why?
nv50: improve broken vbo handling
This change prevents a call to nv50_push_vbo() without a valid state.
This change fixes: "piglit/bin/gl-3.1-vao-broken-attrib -auto -fbo"
Fixes: 1cd455b1 ("gallium: extend draw_vbo to support multi draws")
Signed-off-by: Patrick Lerda patrick9876@free.fr