lima/ppir: turn store_color into ALU node

lima/ppir: turn store_color into ALU node

We don't have a special OP to store color in PP, all we need to do is to
store gl_FragColor into reg0, thus it's just a mov and therefore ALU node.

Yet we still need to indicate that it's store_color op so regalloc ignores
its destination.

Tested-by: Andreas Baierl <ichgeh@imkreisrum.de>
Reviewed-by: Qiang Yu <yuq825@gmail.com>
Reviewed-by: Erico Nunes <nunes.erico@gmail.com>
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>