mesa: Don't call driver when there is no compute work
mesa: Don't call driver when there is no compute work

The ARB_compute_shader spec says:

  "If the work group count in any dimension is zero, no work groups
   are dispatched."

Signed-off-by: Jordan Justen <a79006884ba9cf5f4f7fcbb92a15b208780d8c97@intel.com>
Reviewed-by: Ilia Mirkin <bcbbb7aa705ec3a5fc5824c01a845186c3d62fca@alum.mit.edu>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler