r300_cmdbuf: Always emit INDX_BUFFER immediately after DRAW_INDEX

DRAW_INDEX writes a vertex count to VAP_VF_CNTL. Docs say that behaviour
is undefined (i.e. lockups happen) when this write is not followed by the
right number of vertex indices.

Thus we used to do the wrong thing when drawing across many cliprects was
necessary, because we emitted a sequence
 DRAW_INDEX, DRAW_INDEX, INDX_BUFFER, INDX_BUFFER
instead of
 DRAW_INDEX, INDX_BUFFER, DRAW_INDEX, INDX_BUFFER
The latter is what we're doing now and which ought to be correct.