virgl: Increase the shader transfer buffer by doubling the size

virgl: Increase the shader transfer buffer by doubling the size

With only linearly increasing the size of the shader transfer buffer
the transfer of very large shaders may fail, so with each attempt double
the size of the buffer.

CTS:
  dEQP-GLES31.functional.ssbo.layout.random.all_shared_buffer.48
  for VTK-GL-CTS b5dcfb9c5 and newer

virglrenderer bug:
  https://gitlab.freedesktop.org/virgl/virglrenderer/issues/150

Fixes: a8987b88ff1db4ac00720a9b56c4bc3aeb666537
    virgl: add driver for virtio-gpu 3D (v2)

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3121>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3121>