nvk: Emit NVK_MME_SCRATCH_DRAW_BEGIN as part of flush_ia_state()

We're always storing it in a scratch register for register pressure
reasons anyway.  We may as well just stash it there as a state reg and
we can avoid emitting it all over the place.  This reduces each draw
call to nvk_flush_gfx_state() followed by the actual draw, which is now
independenty of any dynamic state.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30703>
1 file changed