nvc0: treat all draws without color0 broadcast as MRT

nvc0: treat all draws without color0 broadcast as MRT

Per the semi-recently-released NVIDIA docs, when this bit is not
enabled, then the result for RT[0] will be used. So if e.g. only a
single RT is drawn to and it's not RT[2], the results will not be
visible. Fixes
GTF-GL45.gtf33.GL3Tests.explicit_attrib_location.explicit_attrib_location_pipeline
which was failing due to a frag shader outputting only to location=2.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>