etnaviv: implement emit_string_marker

etnaviv: implement emit_string_marker

Writes string to cmdstream in payload of a nop command.
Could be useful for internal driver debugging too.

Here is how it looks decoded:

    0x18000000, /* NOP (3) OP=NOP */
    0x65736572, /*   rese */
    0x18000000, /* NOP (3) OP=NOP */
    0x00000074, /*   t */
    0x00000000, /*   GL.API_MODE := OPENGL */

or

    0x00000705, /*   GL.STALL_TOKEN := FROM=RA,TO=PE,FLIP0=0,FLIP1=0 */
    0x00000001, /*   TS.FLUSH_CACHE := FLUSH=1 */
    0x18000000, /* NOP (3) OP=NOP */
    0x616e7465, /*   etna */
    0x18000000, /* NOP (3) OP=NOP */
    0x6275735f, /*   _sub */
    0x18000000, /* NOP (3) OP=NOP */
    0x5f74696d, /*   mit_ */
    0x18000000, /* NOP (3) OP=NOP */
    0x735f7372, /*   rs_s */
    0x18000000, /* NOP (3) OP=NOP */
    0x65746174, /*   tate */
    0x00004606, /*   RS.CONFIG := SOURCE_FORMAT=A8R8G8B8

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3744>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3744>