gallium: move pipe_draw_info::index_bias to pipe_draw_start_count_bias

gallium: move pipe_draw_info::index_bias to pipe_draw_start_count_bias

this moves index_bias into the multidraw struct, enabling draws where the value
changes to be merged; the draw_info struct member is renamed and moved to the end
of the struct for tc use

u_vbuf still has some checks to split draws if index_bias changes, maybe
this can be removed at some point?

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10166>