radeonsi: fix gl_BaseVertex in non-indexed draws

radeonsi: fix gl_BaseVertex in non-indexed draws

gl_BaseVertex is supposed to be 0 in non-indexed draws. Unfortunately, the
way they're implemented, the VGT always generates indices starting at 0,
and the VS prolog adds the start index.

There's a VGT_INDX_OFFSET register which causes the VGT to start at a
driver-defined index. However, this register cannot be written from
indirect draws.

So fix this unlikely case by setting a bit to tell the VS whether the
draw is indexed or not, so that gl_BaseVertex can be adjusted accordingly
when used.

Fixes a bug in
KHR-GL45.shader_draw_parameters_tests.ShaderMultiDrawArraysParameters.*

Reviewed-by: Marek Olšák <marek.olsak@amd.com>