Stop allocating ri_buf and ri_stamp dynamically. As commented in
rasops.h, it is not safe to use kmem_alloc(9) in rasops_init();
rasops routines can be used for early putchar, which means that
UVM is not fully initialized.

Should fix a problem reported by macallan:
http://mail-index.netbsd.org/tech-kern/2019/08/02/msg025327.html

Instead of using ri_buf, inline function rasops_memcpy32() is
introduced to fill 32bit data efficiently.

Instead of using ri_stamp (per device stamp), stamp_ri is
introduced to distinguish for which device stamp is calculated.

(rin)