gallium: Drop software-only primitive restart support.

gallium: Drop software-only primitive restart support.

The drivers not flagging primitive restart support are r300 swtcl, svga,
nv30, and vc4.

The point of primitive restart is to slightly reduce draw call overhead
for apps by batching multiple draws.  If we do an extra pass to read the
index buffer and split back into multiple draws, we've entirely missed the
point.  This is particularly bad for drivers that otherwise have hardware
IB reads, where the readback is probably uncached.

Reviewed-by: Rob Clark <robdclark@gmail.com>