st/mesa: unmap the stream_uploader buffer before drawing

st/mesa: unmap the stream_uploader buffer before drawing

Some drivers require that the vertex buffers be unmapped prior to
drawing.  This change unmaps the stream_uploader buffer after we've
uploaded the zero-stride attributes (unless the driver supports
rendering with mapped buffers).

This fixes a regression in the VMware driver since 17f776c27be266f2.
Some Mesa demos such as mandelbrot and brick would display black
quads instead of the expected rendering.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>