draw: fix non-indexed draw calls if there's an index buffer

draw: fix non-indexed draw calls if there's an index buffer

pipe_draw_info::indexed determines if it should be indexed and not
the presence of an index buffer.

This fixes crashes in r300g.

NOTE: This is a candidate for the stable branches.

Tested-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Brian Paul <brianp@vmware.com>