nv50, nvc0: clear out RT on a null cbuf

nv50, nvc0: clear out RT on a null cbuf

This is needed since commit 9baa45f78b (st/mesa: bind NULL colorbuffers
as specified by glDrawBuffers).

This implementation is highly based on a larger commit by
Christoph Bumiller <e0425955@student.tuwien.ac.at> in his gallium-nine
branch.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>