radv: Don't explicitly reference vertex shader for draw_id.

radv: Don't explicitly reference vertex shader for draw_id.

With merged shaders the vertex shader may not exist. This got in
because the offending patch was written before merged shaders were
upstream, but committed after.

Fixes: 75dfab24a2c 'radv: refactor indirect draws with radv_draw_info'
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>