gallium/ttn: mark location specially in nir for color0-writes-all

gallium/ttn: mark location specially in nir for color0-writes-all

We need to distinguish a shader that has separate writes to each MRT
from one which is supposed to write the data from MRT 0 to all the MRTs.
In TGSI this is done with a property. NIR doesn't have that, so encode
it as a funny location and decode on the other end.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Eric Anholt <eric@anholt.net>