The guest needs to always mark SSBOs as unclean, since it's possible
the shader can write to them without doing a transfer.

We don't necessarily need to add VIRGL_BIND_SHADER_BUFFER, and we can
do this just with PIPE_BIND_SHADER_BUFFER, but this seemed most
consistent with prior commits.

With some host side changes too, fixes:
    dEQP-GLES31.functional.shaders.builtin_var.compute.num_work_groups

Change-Id: Iafb5ab6bdb4483806709641a5c6f769b0679e636