svga: fix zero-stride vertex array bug

svga: fix zero-stride vertex array bug

For zero-stride vertex arrays, the svga driver copies the value into
the constant value and uses that value in the shader.  The recent
gallium-userbuf changes caused a regression in this.  An example
symptom was per-primitive glColor3f() calls getting ignored.

Where we copied the vertex value from the vertex buffer to the
constant buffer we neglected to take into account the
pipe_vertex_buffer::buffer_offset field.  Adding that value to the
source offset fixes the problem.  Actually, it looks like we should
have been doing this all along, but it never was an issue before for
some reason.