r300g: cache packet dwords of 3D_LOAD_VBPNTR in a command buffer if possible

r300g: cache packet dwords of 3D_LOAD_VBPNTR in a command buffer if possible

It's not always possible to preprocess the content of 3D_LOAD_VBPNTR
in a command buffer, because the offset to all vertex buffers (which
the packet depends on) is derived from the "start" parameter of draw_arrays
and the "indexBias" parameter of draw_elements, but we can at least lazily
make a command buffer for the case when offset == 0, which should occur
most of the time.