freedreno/ir3: move emit_const to ir3

freedreno/ir3: move emit_const to ir3

Details of the cmdstream packets are different between a3xx and a4xx,
but the logic about the layout of const registers is the same, as that
is dictated by the ir3 shader compiler.  So rather than duplicating
logic that is tightly coupled to ir3 between a3xx and a4xx, move this
into ir3 and use per-generation callbacks for to build the cmdstream
packets.

This should make it easier to pass additional const regs (such as for
transform feedback).  And it also keeps the layout internal to ir3 in
case we want to make the layout more dynamic some day.

Signed-off-by: Rob Clark <robclark@freedesktop.org>