nv50: reimplement draw_arrays_instanced(), do draw_arrays() in terms of it

nv50: reimplement draw_arrays_instanced(), do draw_arrays() in terms of it

This makes draw_arrays()/draw_arrays_instanced() do the right thing and
not require the (probably broken anyway) flush_notify() usage.

It also fixes a potential bug in the behaviour of reading InstanceID from
shaders, where 0 should be read for non-instanced drawing, previously it
was possible to read non-0 ids if mixing instanced/non-instanced.